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.util.*;
  import java.lang.*;
  import java.rmi.ConnectIOException;
  import java.io.*;
  
  public class Main {
      static FastReader in;
      static PrintWriter out;
  
      static int bit(long n) {
          return (n == 0) ? 0 : (1 + bit(n & (n - 1)));
      }
  
      static void p(Object o) {
          out.print(o);
      }
  
      static void pn(Object o) {
          out.println(o);
      }
  
      static void pni(Object o) {
          out.println(o);
          out.flush();
      }
  
      static String n() throws Exception {
          return in.next();
      }
  
      static String nln() throws Exception {
          return in.nextLine();
      }
  
      static int ni() throws Exception {
          return Integer.parseInt(in.next());
      }
  
      static long nl() throws Exception {
          return Long.parseLong(in.next());
      }
  
      static double nd() throws Exception {
          return Double.parseDouble(in.next());
      }
  
      static class FastReader {
          static BufferedReader br;
          static StringTokenizer st;
  
          public FastReader() {
              br = new BufferedReader(new InputStreamReader(System.in));
          }
  
          public FastReader(String s) throws Exception {
              br = new BufferedReader(new FileReader(s));
          }
  
          String next() throws Exception {
              while (st == null || !st.hasMoreElements()) {
                  try {
                      st = new StringTokenizer(br.readLine());
                  } catch (IOException e) {
                      throw new Exception(e.toString());
                  }
              }
              return st.nextToken();
          }
  
          String nextLine() throws Exception {
              String str = "";
              try {
                  str = br.readLine();
              } catch (IOException e) {
                  throw new Exception(e.toString());
              }
              return str;
          }
      }
  
      static long power(long a, long b) {
          if (b == 0)
              return 1;
          long val = power(a, b / 2);
          val = val * val;
          if ((b % 2) != 0)
              val = val * a;
          return val;
      }
  
      static long power(long a, long b, long mod) {
          if (b == 0)
              return 1L;
          long val = power(a, b / 2) % mod;
          val = (val * val) % mod;
          if ((b % 2) != 0)
              val = (val * a) % mod;
          return val % mod;
      }
  
      static ArrayList<Long> prime_factors(long n) {
          ArrayList<Long> ans = new ArrayList<Long>();
          while (n % 2 == 0) {
              ans.add(2L);
              n /= 2L;
          }
          for (long i = 3; i <= Math.sqrt(n); i++) {
              while (n % i == 0) {
                  ans.add(i);
                  n /= i;
              }
          }
          if (n > 2) {
              ans.add(n);
          }
          return ans;
      }
  
      static void sort(ArrayList<Long> a) {
          Collections.sort(a);
      }
  
      static void reverse_sort(ArrayList<Long> a) {
          Collections.sort(a, Collections.reverseOrder());
      }
  
      static void swap(long[] a, int i, int j) {
          long temp = a[i];
          a[i] = a[j];
          a[j] = temp;
      }
  
      static void swap(List<Long> a, int i, int j) {
          long temp = a.get(i);
          a.set(j, a.get(i));
          a.set(j, temp);
      }
  
      static void sieve(boolean[] prime) {
          int n = prime.length - 1;
          Arrays.fill(prime, true);
          for (int i = 2; i * i <= n; i++) {
              if (prime[i]) {
                  for (int j = 2 * i; j <= n; j += i) {
                      prime[j] = false;
                  }
              }
          }
      }
  
      static long gcd(long a, long b) {
          if (b == 0)
              return a;
          return gcd(b, a % b);
      }
  
      static long mod = 1000000007;
  
      public static void sort(long[] arr, int l, int r) {
          if (l >= r)
              return;
          int mid = (l + r) / 2;
          sort(arr, l, mid);
          sort(arr, mid + 1, r);
          merge(arr, l, mid, r);
      }
  
      public static void sort(int[] arr, int l, int r) {
          if (l >= r)
              return;
          int mid = (l + r) / 2;
          sort(arr, l, mid);
          sort(arr, mid + 1, r);
          merge(arr, l, mid, r);
      }
  
      static void merge(int[] arr, int l, int mid, int r) {
          int[] left = new int[mid - l + 1];
          int[] right = new int[r - mid];
          for (int i = l; i <= mid; i++) {
              left[i - l] = arr[i];
          }
          for (int i = mid + 1; i <= r; i++) {
              right[i - (mid + 1)] = arr[i];
          }
          int left_start = 0;
          int right_start = 0;
          int left_length = mid - l + 1;
          int right_length = r - mid;
          int temp = l;
          while (left_start < left_length && right_start < right_length) {
              if (left[left_start] < right[right_start]) {
                  arr[temp] = left[left_start++];
              } else {
                  arr[temp] = right[right_start++];
              }
              temp++;
          }
          while (left_start < left_length) {
              arr[temp++] = left[left_start++];
          }
          while (right_start < right_length) {
              arr[temp++] = right[right_start++];
          }
      }
  
      static void merge(long[] arr, int l, int mid, int r) {
          long[] left = new long[mid - l + 1];
          long[] right = new long[r - mid];
          for (int i = l; i <= mid; i++) {
              left[i - l] = arr[i];
          }
          for (int i = mid + 1; i <= r; i++) {
              right[i - (mid + 1)] = arr[i];
          }
          int left_start = 0;
          int right_start = 0;
          int left_length = mid - l + 1;
          int right_length = r - mid;
          int temp = l;
          while (left_start < left_length && right_start < right_length) {
              if (left[left_start] < right[right_start]) {
                  arr[temp] = left[left_start++];
              } else {
                  arr[temp] = right[right_start++];
              }
              temp++;
          }
          while (left_start < left_length) {
              arr[temp++] = left[left_start++];
          }
          while (right_start < right_length) {
              arr[temp++] = right[right_start++];
          }
      }
  
      static class pair implements Comparable<pair> {
          int a, b;
  
          public pair(int a, int b) {
              this.a = a;
              this.b = b;
          }
  
          public int compareTo(pair p) {
              if (this.a == p.a)
                  return this.b - p.b;
              return this.a - p.a;
          }
  
      }
  
      static HashMap<Long, Integer> map_prime_factors(long n) {
          HashMap<Long, Integer> map = new HashMap<>();
          while (n % 2 == 0) {
              map.put(2L, map.getOrDefault(2L, 0) + 1);
              n /= 2L;
          }
          for (long i = 3; i <= Math.sqrt(n); i++) {
              while (n % i == 0) {
                  map.put(i, map.getOrDefault(i, 0) + 1);
                  n /= i;
              }
          }
          if (n > 2) {
              map.put(n, map.getOrDefault(n, 0) + 1);
          }
          return map;
      }
  
      static int count = 1;
      static int[] p = new int[100002];
      static long[] flat_tree = new long[300002];
      static int[] in_time = new int[1000002];
      static int[] out_time = new int[1000002];
      static long[] subtree_gcd = new long[100002];
      static int w = 0;
      static boolean poss = true;
  
      public static void main(String[] args) throws Exception {
          in = new FastReader();
          out = new PrintWriter(System.out);
          int t = ni();
          while (t-- > 0) {
              String a = n();
              String b = n();
              int i = a.length() - 1;
              int j = b.length() - 1;
              String ans = "";
              boolean poss = true;
              while (i >= 0 && j >= 0) {
                  if (a.charAt(i) == b.charAt(j)) {
                      ans = "0"+ans;
                      i--;
                      j--;
                  } else {
                      if (a.charAt(i) < b.charAt(j)) {
                          int val = b.charAt(j) - a.charAt(i);
                          ans = String.valueOf(val)+ans;
                          i--;
                          j--;
                      } else {
                          int val = 10 + (b.charAt(j) - '0') - (a.charAt(i) - '0');
                          String arr = String.valueOf((a.charAt(i) - '0') + val);
                          if (j - 1 >= 0 && b.charAt(j - 1) == arr.charAt(0)) {
                              i--;
                              j -= 2;
                              ans = String.valueOf(val) + ans;
                          } else {
                              poss = false;
                              break;
                          }
  
                      }
                  }
              }
              if (!poss ||(i>=0))
                  pn("-1");
              else{
                  if(j>=0){
                      pn(b.substring(0, j+1)+ans);
                  }else pn(Integer.parseInt(ans));
              }
          }
          out.flush();
          out.close();
      }
  
      static void find(int l, int r, int level, int[] arr, int[] depth) {
          if (r < l)
              return;
          int max = find(l, r, arr);
          depth[max] = level;
          find(l, max - 1, level + 1, arr, depth);
          find(max + 1, r, level + 1, arr, depth);
      }
  
      static int find(int l, int r, int[] arr) {
          int index = l;
          for (int i = l + 1; i <= r; i++) {
              if (arr[index] < arr[i]) {
                  index = i;
              }
          }
          return index;
      }
  
      static long start(int[] a, int k) {
          if (a.length == 0)
              return 0;
          if (a.length <= k)
              return 2 * a[a.length - 1];
          long ans = 0;
          int i = 0;
          for (i = k - 1; i < a.length; i += k) {
              ans += 2 * a[i];
          }
          i -= k;
          if (i != a.length - 1) {
              return ans + 2 * a[a.length - 1];
          } else
              return ans;
      }
  
      static long end(int[] a, int k) {
          if (a.length == 0)
              return 0;
          if (a.length <= k)
              return a[a.length - 1];
          if (k == 1) {
              long sum = 0;
              for (int i = 0; i < a.length - 1; i++)
                  sum += 2 * a[i];
              return sum + a[a.length - 1];
          }
          long ans = 0;
          int i = 0;
          for (i = k - 1; i < a.length; i += k) {
              if (i != a.length - 1)
                  ans += 2 * a[i];
          }
          i -= k;
          return ans + a[a.length - 1];
      }
  
      static void print(int[] a) {
          for (int i = 0; i < a.length; i++)
              p(a[i] + " ");
          pn("");
      }
  
      static long count(long n) {
          long count = 0;
          while (n != 0) {
              n /= 10;
              count++;
          }
          return count;
      }
  
      static void swap(long a, long b) {
          long temp = a;
          a = b;
          b = temp;
      }
  
      static void swap(int[] a, int i, int j) {
          int temp = a[i];
          a[i] = a[j];
          a[j] = temp;
      }
  
      static int LcsOfPrefix(String a, String b) {
          int i = 0;
          int j = 0;
          int count = 0;
          while (i < a.length() && j < b.length()) {
              if (a.charAt(i) == b.charAt(j)) {
                  j++;
                  count++;
              }
              i++;
          }
          return a.length() + b.length() - 2 * count;
      }
  
      static void reverse(int[] a, int n) {
          for (int i = 0; i < n / 2; i++) {
              int temp = a[i];
              a[i] = a[n - i - 1];
              a[n - i - 1] = temp;
          }
      }
  
      static char get_char(int a) {
          return (char) (a + 'a');
      }
  
      static void dfs(int i, List<List<Integer>> arr, boolean[] visited, int parent) {
          visited[i] = true;
          for (int v : arr.get(i)) {
              if (!visited[v]) {
                  dfs(v, arr, visited, i);
              }
          }
      }
  
      static int find1(List<Integer> a, int val) {
          int ans = -1;
          int l = 0;
          int r = a.size() - 1;
          while (l <= r) {
              int mid = l + (r - l) / 2;
              if (a.get(mid) >= val) {
                  r = mid - 1;
                  ans = mid;
              } else
                  l = mid + 1;
          }
          return ans;
      }
  
      static int find2(List<Integer> a, int val) {
          int l = 0;
          int r = a.size() - 1;
          int ans = -1;
          while (l <= r) {
              int mid = l + (r - l) / 2;
              if (a.get(mid) <= val) {
                  ans = mid;
                  l = mid + 1;
              } else
                  r = mid - 1;
          }
          return ans;
      }
  
      static void dfs(List<List<Integer>> arr, int node, int parent, long[] val) {
          p[node] = parent;
          in_time[node] = count;
          flat_tree[count] = val[node];
          subtree_gcd[node] = val[node];
          count++;
          for (int adj : arr.get(node)) {
              if (adj == parent)
                  continue;
              dfs(arr, adj, node, val);
              subtree_gcd[node] = gcd(subtree_gcd[adj], subtree_gcd[node]);
          }
          out_time[node] = count;
          flat_tree[count] = val[node];
          count++;
      }
  }