description: Tanya is learning how to add numbers, but so far she is not doing it correctly. She is adding two numbers a and b using the following algorithm:
  
    1. If one of the numbers is shorter than the other, Tanya adds leading zeros so that the numbers are the same length. 
    2. The numbers are processed from right to left (that is, from the least significant digits to the most significant). 
    3. In the first step, she adds the last digit of a to the last digit of b and writes their sum in the answer. 
    4. At each next step, she performs the same operation on each pair of digits in the same place and writes the result to the left side of the answer. 
  
  
  
  For example, the numbers a = 17236 and b = 3465 Tanya adds up as follows:
  
  $$$ \large{ \begin{array}{r} + \begin{array}{r} 17236\\\ 03465\\\ \end{array} \\\ \hline \begin{array}{r} 1106911 \end{array} \end{array}} $$$
  
    * calculates the sum of 6 + 5 = 11 and writes 11 in the answer. 
    * calculates the sum of 3 + 6 = 9 and writes the result to the left side of the answer to get 911. 
    * calculates the sum of 2 + 4 = 6 and writes the result to the left side of the answer to get 6911. 
    * calculates the sum of 7 + 3 = 10, and writes the result to the left side of the answer to get 106911. 
    * calculates the sum of 1 + 0 = 1 and writes the result to the left side of the answer and get 1106911. 
  
  
  
  As a result, she gets 1106911.
  
  You are given two positive integers a and s. Find the number b such that by adding a and b as described above, Tanya will get s. Or determine that no suitable b exists.
  
  Input
  
  The first line of input data contains an integer t (1 ≤ t ≤ 10^4) — the number of test cases.
  
  Each test case consists of a single line containing two positive integers a and s (1 ≤ a < s ≤ 10^{18}) separated by a space.
  
  Output
  
  For each test case print the answer on a separate line.
  
  If the solution exists, print a single positive integer b. The answer must be written without leading zeros. If multiple answers exist, print any of them.
  
  If no suitable number b exists, output -1.
  
  Example
  
  Input
  
  
  6
  17236 1106911
  1 5
  108 112
  12345 1023412
  1 11
  1 20
  
  
  Output
  
  
  3465
  4
  -1
  90007
  10
  -1
  
  Note
  
  The first test case is explained in the main part of the statement.
  
  In the third test case, we cannot choose b that satisfies the problem statement.
solution: import java.math.BigInteger;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.Collections;
  import java.util.HashSet;
  import java.util.List;
  import java.util.Set;
  import java.util.Stack;
  
  public class Main {
      static long MOD = 998244353l;
      int min = Integer.MAX_VALUE;
      boolean re = false;
  
      public static void main(String[] args) throws Exception {
          // FileInputStream fis = new FileInputStream(new
          // File("/Users/goudezhao/Public/JavaTest/1.txt"));
          var sc = new FastScanner();
          // var sc = new FastScanner(fis);
          // var pw = new FastPrintStream("/Users/goudezhao/Public/JavaTest/1.out");
          var pw = new FastPrintStream();
          solve(sc, pw);
          sc.close();
          pw.flush();
          pw.close();
      }
  
      public static void solve(FastScanner sc, FastPrintStream pw) {
          int t = sc.nextInt();
          List<Long> list = new ArrayList<Long>();
          Set<Long> set = new HashSet<Long>();
          for (long i = 1; i * i <= 1000000000; i++) {
              set.add(i * i);
          }
          for (long i = 1; i * i * i <= 1000000000; i++) {
              set.add(i * i * i);
          }
          // pw.println(set.size());
          for (Long l : set) {
              list.add(l);
          }
          Collections.sort(list);
          for (int times = 0; times < t; times++) {
              long a = sc.nextLong();
              long s = sc.nextLong();
              StringBuffer sb = new StringBuffer();
              boolean ok = true;
              while (a > 0) {
                  if (a % 10 <= s % 10) {
                      sb.append((s % 10 - a % 10));
                      a = a / 10;
                      s = s / 10;
                  } else {
                      long temp = s % 100;
                      if (temp >= 20 || temp < 10) {
                          pw.println(-1);
                          ok = false;
                          break;
                      }
                      if (temp - a >= 10) {
                          pw.println(-1);
                          ok = false;
                          break;
                      }
                      sb.append(temp - a % 10);
                      a = a / 10;
                      s = s / 100;
                  }
              }
              if (ok) {
                  while (s>0) {
                      sb.append(s%10);
                      s/=10;
                  }
                  char result[] = sb.toString().toCharArray();
                  int index= result.length-1;
                  while (index>=0&&result[index]=='0') {
                      index--;
                  }
                  while (index>=0) {
                      pw.print(result[index]);
                      index--;
                  }
                  pw.println();
              }
          }
      }
  
      public static void swap(int[] s, int i, int j) {
          int tmp = s[i];
          s[i] = s[j];
          s[j] = tmp;
      }
  
      public void permutation(int[] s, int from, int to, Point a[], Point b[]) {
          if (to <= 1)
              return;
          if (from == to) {
              check(s, a, b);
          } else {
              for (int i = from; i <= to; i++) {
                  swap(s, i, from); // 交换前缀，使其产生下一个前缀
                  permutation(s, from + 1, to, a, b);
                  swap(s, from, i); // 将前缀换回，继续做上一个前缀的排列
              }
          }
      }
  
      public void check(int[] s, Point a[], Point b[]) {
          Point tempa[] = new Point[a.length];
          Point tempb[] = new Point[b.length];
          for (int i = 0; i < a.length; i++) {
              tempa[i] = new Point();
              tempa[i].x = Math.min(a[i].x, a[i].y);
              tempa[i].y = Math.max(a[i].x, a[i].y);
          }
          for (int i = 0; i < a.length; i++) {
              tempb[i] = new Point();
              tempb[i].x = Math.min(s[b[i].x - 1], s[b[i].y - 1]);
              tempb[i].y = Math.max(s[b[i].x - 1], s[b[i].y - 1]);
          }
          Arrays.sort(tempa);
          Arrays.sort(tempb);
          for (int i = 0; i < a.length; i++) {
              if (tempa[i].x != tempb[i].x || tempa[i].y != tempb[i].y) {
                  return;
              }
          }
          this.re = true;
      }
  
      public static long anothertoTen(long ano, int another) {
          long ten = 0;
          long now = 1;
          long temp = ano;
          while (temp > 0) {
              long i = temp % 10;
              ten += now * i;
              now *= another;
              temp /= 10;
          }
          return ten;
      }
  
      public static long tentoAnother(long ten, int another) {
          Stack<Long> stack = new Stack<Long>();
          while (ten > 0) {
              stack.add(ten % another);
              ten /= another;
          }
          long re = 0;
          while (!stack.isEmpty()) {
              long pop = stack.pop();
              re = re * 10 + pop;
          }
          return re;
      }
  
      // 2C5 = 5*4/(2*1)
      public static long XCY(long x, long y) {
          long temp = 1;
          for (int i = 0; i < x; i++) {
              temp = (temp * (y - i)) % MOD;
          }
          long tempx = 1;
          for (int i = 2; i <= x; i++) {
              tempx = (tempx * i) % MOD;
          }
          tempx = modpow(tempx, (long) MOD - 2);
          temp = (temp * tempx) % MOD;
          return temp;
      }
  
      static long modpow(long N, Long K) {
          return BigInteger.valueOf(N).modPow(BigInteger.valueOf(K), BigInteger.valueOf(MOD)).longValue();
      }
  
      static long modpow(long N, Long K, long mod) {
          return BigInteger.valueOf(N).modPow(BigInteger.valueOf(K), BigInteger.valueOf(mod)).longValue();
      }
  
      public static long gcd(long a, long b) {
          if (b == 0) {
              return a;
          }
          if (a < b) {
              return gcd(b, a);
          }
          return gcd(b, a % b);
      }
  
      public static int gcd(int a, int b) {
          if (b == 0) {
              return a;
          }
          if (a < b) {
              return gcd(b, a);
          }
          return gcd(b, a % b);
      }
  
  }
  
  class Range {
      long start = 0;
      long end = 0;
  }
  
  class Point implements Comparable {
      int x;
      int y;
  
      public int compareTo(Object p) {
          Point t = (Point) p;
          if (t.x != this.x) {
              return t.x - this.x;
          }
          return t.y - this.y;
      }
  }
  
  class PointY implements Comparable {
      int index;
      long a;
      long b;
  
      public int compareTo(Object p) {
          PointY t = (PointY) p;
          if (this.a > t.a) {
              return -1;
          }
          if (this.a < t.a) {
              return 1;
          }
          return 0;
      }
  }
  
  class FastPrintStream implements AutoCloseable {
      private static final int BUF_SIZE = 1 << 15;
      private final byte[] buf = new byte[BUF_SIZE];
      private int ptr = 0;
      private final java.lang.reflect.Field strField;
      private final java.nio.charset.CharsetEncoder encoder;
  
      private java.io.OutputStream out;
  
      public FastPrintStream(java.io.OutputStream out) {
          this.out = out;
          java.lang.reflect.Field f;
          try {
              f = java.lang.String.class.getDeclaredField("value");
              f.setAccessible(true);
          } catch (NoSuchFieldException | SecurityException e) {
              f = null;
          }
          this.strField = f;
          this.encoder = java.nio.charset.StandardCharsets.US_ASCII.newEncoder();
      }
  
      public FastPrintStream(java.io.File file) throws java.io.IOException {
          this(new java.io.FileOutputStream(file));
      }
  
      public FastPrintStream(java.lang.String filename) throws java.io.IOException {
          this(new java.io.File(filename));
      }
  
      public FastPrintStream() {
          this(System.out);
          try {
              java.lang.reflect.Field f = java.io.PrintStream.class.getDeclaredField("autoFlush");
              f.setAccessible(true);
              f.set(System.out, false);
          } catch (IllegalAccessException | IllegalArgumentException | NoSuchFieldException e) {
              // ignore
          }
      }
  
      public FastPrintStream println() {
          if (ptr == BUF_SIZE)
              internalFlush();
          buf[ptr++] = (byte) '\n';
          return this;
      }
  
      public FastPrintStream println(java.lang.Object o) {
          return print(o).println();
      }
  
      public FastPrintStream println(java.lang.String s) {
          return print(s).println();
      }
  
      public FastPrintStream println(char[] s) {
          return print(s).println();
      }
  
      public FastPrintStream println(char c) {
          return print(c).println();
      }
  
      public FastPrintStream println(int x) {
          return print(x).println();
      }
  
      public FastPrintStream println(long x) {
          return print(x).println();
      }
  
      public FastPrintStream println(double d, int precision) {
          return print(d, precision).println();
      }
  
      private FastPrintStream print(byte[] bytes) {
          int n = bytes.length;
          if (ptr + n > BUF_SIZE) {
              internalFlush();
              try {
                  out.write(bytes);
              } catch (java.io.IOException e) {
                  throw new RuntimeException();
              }
          } else {
              System.arraycopy(bytes, 0, buf, ptr, n);
              ptr += n;
          }
          return this;
      }
  
      public FastPrintStream print(java.lang.Object o) {
          return print(o.toString());
      }
  
      public FastPrintStream print(java.lang.String s) {
          if (strField == null) {
              return print(s.getBytes());
          } else {
              try {
                  return print((byte[]) strField.get(s));
              } catch (IllegalAccessException e) {
                  return print(s.getBytes());
              }
          }
      }
  
      public FastPrintStream print(char[] s) {
          try {
              return print(encoder.encode(java.nio.CharBuffer.wrap(s)).array());
          } catch (java.nio.charset.CharacterCodingException e) {
              byte[] bytes = new byte[s.length];
              for (int i = 0; i < s.length; i++) {
                  bytes[i] = (byte) s[i];
              }
              return print(bytes);
          }
      }
  
      public FastPrintStream print(char c) {
          if (ptr == BUF_SIZE)
              internalFlush();
          buf[ptr++] = (byte) c;
          return this;
      }
  
      public FastPrintStream print(int x) {
          if (x == 0) {
              if (ptr == BUF_SIZE)
                  internalFlush();
              buf[ptr++] = '0';
              return this;
          }
          int d = len(x);
          if (ptr + d > BUF_SIZE)
              internalFlush();
          if (x < 0) {
              buf[ptr++] = '-';
              x = -x;
              d--;
          }
          int j = ptr += d;
          while (x > 0) {
              buf[--j] = (byte) ('0' + (x % 10));
              x /= 10;
          }
          return this;
      }
  
      public FastPrintStream print(long x) {
          if (x == 0) {
              if (ptr == BUF_SIZE)
                  internalFlush();
              buf[ptr++] = '0';
              return this;
          }
          int d = len(x);
          if (ptr + d > BUF_SIZE)
              internalFlush();
          if (x < 0) {
              buf[ptr++] = '-';
              x = -x;
              d--;
          }
          int j = ptr += d;
          while (x > 0) {
              buf[--j] = (byte) ('0' + (x % 10));
              x /= 10;
          }
          return this;
      }
  
      public FastPrintStream print(double d, int precision) {
          if (d < 0) {
              print('-');
              d = -d;
          }
          d += Math.pow(10, -d) / 2;
          print((long) d).print('.');
          d -= (long) d;
          for (int i = 0; i < precision; i++) {
              d *= 10;
              print((int) d);
              d -= (int) d;
          }
          return this;
      }
  
      private void internalFlush() {
          try {
              out.write(buf, 0, ptr);
              ptr = 0;
          } catch (java.io.IOException e) {
              throw new RuntimeException(e);
          }
      }
  
      public void flush() {
          try {
              out.write(buf, 0, ptr);
              out.flush();
              ptr = 0;
          } catch (java.io.IOException e) {
              throw new RuntimeException(e);
          }
      }
  
      public void close() {
          try {
              out.close();
          } catch (java.io.IOException e) {
              throw new RuntimeException(e);
          }
      }
  
      private static int len(int x) {
          int d = 1;
          if (x >= 0) {
              d = 0;
              x = -x;
          }
          int p = -10;
          for (int i = 1; i < 10; i++, p *= 10)
              if (x > p)
                  return i + d;
          return 10 + d;
      }
  
      private static int len(long x) {
          int d = 1;
          if (x >= 0) {
              d = 0;
              x = -x;
          }
          long p = -10;
          for (int i = 1; i < 19; i++, p *= 10)
              if (x > p)
                  return i + d;
          return 19 + d;
      }
  }
  
  class FastScanner implements AutoCloseable {
      private final java.io.InputStream in;
      private final byte[] buf = new byte[2048];
      private int ptr = 0;
      private int buflen = 0;
  
      public FastScanner(java.io.InputStream in) {
          this.in = in;
      }
  
      public FastScanner() {
          this(System.in);
      }
  
      private boolean hasNextByte() {
          if (ptr < buflen)
              return true;
          ptr = 0;
          try {
              buflen = in.read(buf);
          } catch (java.io.IOException e) {
              throw new RuntimeException(e);
          }
          return buflen > 0;
      }
  
      private int readByte() {
          return hasNextByte() ? buf[ptr++] : -1;
      }
  
      public boolean hasNext() {
          while (hasNextByte() && !(32 < buf[ptr] && buf[ptr] < 127))
              ptr++;
          return hasNextByte();
      }
  
      private StringBuilder nextSequence() {
          if (!hasNext())
              throw new java.util.NoSuchElementException();
          StringBuilder sb = new StringBuilder();
          for (int b = readByte(); 32 < b && b < 127; b = readByte()) {
              sb.appendCodePoint(b);
          }
          return sb;
      }
  
      public String next() {
          return nextSequence().toString();
      }
  
      public String next(int len) {
          return new String(nextChars(len));
      }
  
      public char nextChar() {
          if (!hasNextByte())
              throw new java.util.NoSuchElementException();
          return (char) readByte();
      }
  
      public char[] nextChars() {
          StringBuilder sb = nextSequence();
          int l = sb.length();
          char[] dst = new char[l];
          sb.getChars(0, l, dst, 0);
          return dst;
      }
  
      public char[] nextChars(int len) {
          if (!hasNext())
              throw new java.util.NoSuchElementException();
          char[] s = new char[len];
          int i = 0;
          int b = readByte();
          while (32 < b && b < 127 && i < len) {
              s[i++] = (char) b;
              b = readByte();
          }
          if (i != len) {
              throw new java.util.NoSuchElementException(
                      String.format("Next token has smaller length than expected.", len));
          }
          return s;
      }
  
      public long nextLong() {
          if (!hasNext())
              throw new java.util.NoSuchElementException();
          long n = 0;
          boolean minus = false;
          int b = readByte();
          if (b == '-') {
              minus = true;
              b = readByte();
          }
          if (b < '0' || '9' < b)
              throw new NumberFormatException();
          while (true) {
              if ('0' <= b && b <= '9') {
                  n = n * 10 + b - '0';
              } else if (b == -1 || !(32 < b && b < 127)) {
                  return minus ? -n : n;
              } else
                  throw new NumberFormatException();
              b = readByte();
          }
      }
  
      public int nextInt() {
          return Math.toIntExact(nextLong());
      }
  
      public double nextDouble() {
          return Double.parseDouble(next());
      }
  
      public void close() {
          try {
              in.close();
          } catch (java.io.IOException e) {
              throw new RuntimeException(e);
          }
      }
  
  }
  
  /**
   * @verified https://atcoder.jp/contests/practice2/tasks/practice2_j
   */
  class SegTree<S> {
      final int MAX;
  
      final int N;
      final java.util.function.BinaryOperator<S> op;
      final S E;
  
      final S[] data;
  
      @SuppressWarnings("unchecked")
      public SegTree(int n, java.util.function.BinaryOperator<S> op, S e) {
          this.MAX = n;
          int k = 1;
          while (k < n)
              k <<= 1;
          this.N = k;
          this.E = e;
          this.op = op;
          this.data = (S[]) new Object[N << 1];
          java.util.Arrays.fill(data, E);
      }
  
      public SegTree(S[] dat, java.util.function.BinaryOperator<S> op, S e) {
          this(dat.length, op, e);
          build(dat);
      }
  
      private void build(S[] dat) {
          int l = dat.length;
          System.arraycopy(dat, 0, data, N, l);
          for (int i = N - 1; i > 0; i--) {
              data[i] = op.apply(data[i << 1 | 0], data[i << 1 | 1]);
          }
      }
  
      public void set(int p, S x) {
          exclusiveRangeCheck(p);
          data[p += N] = x;
          p >>= 1;
          while (p > 0) {
              data[p] = op.apply(data[p << 1 | 0], data[p << 1 | 1]);
              p >>= 1;
          }
      }
  
      public S get(int p) {
          exclusiveRangeCheck(p);
          return data[p + N];
      }
  
      public S prod(int l, int r) {
          if (l > r) {
              throw new IllegalArgumentException(String.format("Invalid range: [%d, %d)", l, r));
          }
          inclusiveRangeCheck(l);
          inclusiveRangeCheck(r);
          S sumLeft = E;
          S sumRight = E;
          l += N;
          r += N;
          while (l < r) {
              if ((l & 1) == 1)
                  sumLeft = op.apply(sumLeft, data[l++]);
              if ((r & 1) == 1)
                  sumRight = op.apply(data[--r], sumRight);
              l >>= 1;
              r >>= 1;
          }
          return op.apply(sumLeft, sumRight);
      }
  
      public S allProd() {
          return data[1];
      }
  
      public int maxRight(int l, java.util.function.Predicate<S> f) {
          inclusiveRangeCheck(l);
          if (!f.test(E)) {
              throw new IllegalArgumentException("Identity element must satisfy the condition.");
          }
          if (l == MAX)
              return MAX;
          l += N;
          S sum = E;
          do {
              l >>= Long.numberOfTrailingZeros(l);
              if (!f.test(op.apply(sum, data[l]))) {
                  while (l < N) {
                      l = l << 1;
                      if (f.test(op.apply(sum, data[l]))) {
                          sum = op.apply(sum, data[l]);
                          l++;
                      }
                  }
                  return l - N;
              }
              sum = op.apply(sum, data[l]);
              l++;
          } while ((l & -l) != l);
          return MAX;
      }
  
      public int minLeft(int r, java.util.function.Predicate<S> f) {
          inclusiveRangeCheck(r);
          if (!f.test(E)) {
              throw new IllegalArgumentException("Identity element must satisfy the condition.");
          }
          if (r == 0)
              return 0;
          r += N;
          S sum = E;
          do {
              r--;
              while (r > 1 && (r & 1) == 1)
                  r >>= 1;
              if (!f.test(op.apply(data[r], sum))) {
                  while (r < N) {
                      r = r << 1 | 1;
                      if (f.test(op.apply(data[r], sum))) {
                          sum = op.apply(data[r], sum);
                          r--;
                      }
                  }
                  return r + 1 - N;
              }
              sum = op.apply(data[r], sum);
          } while ((r & -r) != r);
          return 0;
      }
  
      private void exclusiveRangeCheck(int p) {
          if (p < 0 || p >= MAX) {
              throw new IndexOutOfBoundsException(
                      String.format("Index %d out of bounds for the range [%d, %d).", p, 0, MAX));
          }
      }
  
      private void inclusiveRangeCheck(int p) {
          if (p < 0 || p > MAX) {
              throw new IndexOutOfBoundsException(
                      String.format("Index %d out of bounds for the range [%d, %d].", p, 0, MAX));
          }
      }
  
      // **************** DEBUG **************** //
  
      private int indent = 6;
  
      public void setIndent(int newIndent) {
          this.indent = newIndent;
      }
  
      @Override
      public String toString() {
          return toSimpleString();
      }
  
      public String toDetailedString() {
          return toDetailedString(1, 0);
      }
  
      private String toDetailedString(int k, int sp) {
          if (k >= N)
              return indent(sp) + data[k];
          String s = "";
          s += toDetailedString(k << 1 | 1, sp + indent);
          s += "\n";
          s += indent(sp) + data[k];
          s += "\n";
          s += toDetailedString(k << 1 | 0, sp + indent);
          return s;
      }
  
      private static String indent(int n) {
          StringBuilder sb = new StringBuilder();
          while (n-- > 0)
              sb.append(' ');
          return sb.toString();
      }
  
      public String toSimpleString() {
          StringBuilder sb = new StringBuilder();
          sb.append('[');
          for (int i = 0; i < N; i++) {
              sb.append(data[i + N]);
              if (i < N - 1)
                  sb.append(',').append(' ');
          }
          sb.append(']');
          return sb.toString();
      }
  }
  
  class DSU {
      private int n;
      private int[] parentOrSize;
  
      public DSU(int n) {
          this.n = n;
          this.parentOrSize = new int[n];
          Arrays.fill(parentOrSize, -1);
      }
  
      int merge(int a, int b) {
          if (!(0 <= a && a < n) || !(0 <= b && b < n)) {
              return -1;
          }
          int x = leader(a);
          int y = leader(b);
          if (x == y)
              return x;
          if (-parentOrSize[x] < -parentOrSize[y]) {
              int tmp = x;
              x = y;
              y = tmp;
          }
          parentOrSize[x] += parentOrSize[y];
          parentOrSize[y] = x;
          return x;
      }
  
      boolean same(int a, int b) {
          if (!(0 <= a && a < n) || !(0 <= b && b < n)) {
              return false;
          }
          return leader(a) == leader(b);
      }
  
      int leader(int a) {
          if (parentOrSize[a] < 0) {
              return a;
          } else {
              parentOrSize[a] = leader(parentOrSize[a]);
              return parentOrSize[a];
          }
      }
  
      int size(int a) {
          if (!(0 <= a && a < n)) {
              return -1;
          }
          return -parentOrSize[leader(a)];
      }
  
      ArrayList<ArrayList<Integer>> groups() {
          int[] leaderBuf = new int[n];
          int[] groupSize = new int[n];
          for (int i = 0; i < n; i++) {
              leaderBuf[i] = leader(i);
              groupSize[leaderBuf[i]]++;
          }
          ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
          for (int i = 0; i < n; i++) {
              result.add(new ArrayList<>());
          }
          for (int i = 0; i < n; i++) {
              result.get(leaderBuf[i]).add(i);
          }
          return result;
      }
  }
  