description: Grandma Capa has decided to knit a scarf and asked Grandpa Sher to make a pattern for it, a pattern is a string consisting of lowercase English letters. Grandpa Sher wrote a string s of length n.
  
  Grandma Capa wants to knit a beautiful scarf, and in her opinion, a beautiful scarf can only be knit from a string that is a palindrome. She wants to change the pattern written by Grandpa Sher, but to avoid offending him, she will choose one lowercase English letter and erase some (at her choice, possibly none or all) occurrences of that letter in string s.
  
  She also wants to minimize the number of erased symbols from the pattern. Please help her and find the minimum number of symbols she can erase to make string s a palindrome, or tell her that it's impossible. Notice that she can only erase symbols equal to the one letter she chose.
  
  A string is a palindrome if it is the same from the left to the right and from the right to the left. For example, the strings 'kek', 'abacaba', 'r' and 'papicipap' are palindromes, while the strings 'abb' and 'iq' are not.
  
  Input
  
  The first line contains a single integer t (1 ≤ t ≤ 100) — the number of test cases. The next 2 ⋅ t lines contain the description of test cases. The description of each test case consists of two lines.
  
  The first line of each test case contains a single integer n (1 ≤ n ≤ 10^5) — the length of the string.
  
  The second line of each test case contains the string s consisting of n lowercase English letters.
  
  It is guaranteed that the sum of n over all test cases does not exceed 2 ⋅ 10^5.
  
  Output
  
  For each test case print the minimum number of erased symbols required to make the string a palindrome, if it is possible, and -1, if it is impossible.
  
  Example
  
  Input
  
  
  5
  8
  abcaacab
  6
  xyzxyz
  4
  abba
  8
  rprarlap
  10
  khyyhhyhky
  
  
  Output
  
  
  2
  -1
  0
  3
  2
  
  Note
  
  In the first test case, you can choose a letter 'a' and erase its first and last occurrences, you will get a string 'bcaacb', which is a palindrome. You can also choose a letter 'b' and erase all its occurrences, you will get a string 'acaaca', which is a palindrome as well.
  
  In the second test case, it can be shown that it is impossible to choose a letter and erase some of its occurrences to get a palindrome.
  
  In the third test case, you don't have to erase any symbols because the string is already a palindrome.
solution: import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.PrintStream;
  import java.util.Arrays;
  import java.io.UncheckedIOException;
  import java.io.Closeable;
  import java.io.Writer;
  import java.io.OutputStreamWriter;
  import java.util.InputMismatchException;
  
  /*
  @author : sanskarXrawat
  @date   : 10/27/2021
  @time   : 9:33 PM
   */
  @SuppressWarnings("ALL")
  public class GrandmaKapa {
  
  
      public static void main(String[] args) throws Throwable {
          Thread thread = new Thread (null, new TaskAdapter (), "", 1 << 29);
          thread.start ();
          thread.join ();
      }
  
      static class TaskAdapter implements Runnable {
          @Override
          public void run() {
              InputStream inputStream = System.in;
              OutputStream outputStream = System.out;
              FastInput in = new FastInput (inputStream);
              FastOutput out = new FastOutput (outputStream);
              Solution solver = new Solution ();
              solver.solve (1, in, out);
              in.close ();
              out.close ();
          }
      }
  
      @SuppressWarnings("unused")
      static class Solution {
          static final Debug debug = new Debug (true);
  
          public void solve(int testNumber, FastInput in, FastOutput out) {
              int test=in.ri ();
  
              while(test-->0){
                  int n=in.ri ();
                  char[] s=in.readString ().toCharArray ();
                  long ans=Long.MAX_VALUE;
  
                  skip:for(char i='a';i<='z';i++){
                      int l=0,h=n-1,counter=0;
                      while (l<h){
                       if(s[l]==s[h]){
                           l++;
                           h--;
                           continue;
                       }
                       if(s[l]!=i && s[h]!=i){
                           continue skip;
                       }
                       counter++;
                       if(s[l]==i)
                           l++;
                       else
                           h--;
                      }
                      ans=StrictMath.min (ans,counter);
                  }
                  out.prtl (ans!=Long.MAX_VALUE?ans:-1);
              }
  
  
          }
  
      }
  
  
      static class FastOutput implements AutoCloseable, Closeable, Appendable {
          private final StringBuilder cache = new StringBuilder (THRESHOLD * 2);
          private static final int THRESHOLD = 32 << 10;
          private final Writer os;
  
          public FastOutput append(CharSequence csq) {
              cache.append (csq);
              return this;
          }
  
          public FastOutput append(CharSequence csq, int start, int end) {
              cache.append (csq, start, end);
              return this;
          }
  
          private void afterWrite() {
              if (cache.length () < THRESHOLD) {
                  return;
              }
              flush ();
          }
  
          public FastOutput(Writer os) {
              this.os = os;
          }
  
          public FastOutput(OutputStream os) {
              this (new OutputStreamWriter (os));
          }
  
          public FastOutput append(char c) {
              cache.append (c);
              afterWrite ();
              return this;
          }
  
          public FastOutput append(String c) {
              cache.append (c);
              afterWrite ();
              return this;
          }
  
          public FastOutput println(String c) {
              return append (c).println ();
          }
  
          public FastOutput println() {
              return append ('\n');
          }
  
          final <T> void prt(T a) {
              append (a + " ");
          }
  
          final <T> void prtl(T a) {
              append (a + "\n");
          }
  
          public FastOutput flush() {
              try {
                  os.append (cache);
                  os.flush ();
                  cache.setLength (0);
              } catch (IOException e) {
                  throw new UncheckedIOException (e);
              }
              return this;
          }
  
          public void close() {
              flush ();
              try {
                  os.close ();
              } catch (IOException e) {
                  throw new UncheckedIOException (e);
              }
          }
  
          public String toString() {
              return cache.toString ();
          }
  
          public FastOutput printf(String format, Object... args) {
              return append (String.format (format, args));
          }
  
          private static int countDigits(long l) {
              if (l >= 1000000000000000000L) return 19;
              if (l >= 100000000000000000L) return 18;
              if (l >= 10000000000000000L) return 17;
              if (l >= 1000000000000000L) return 16;
              if (l >= 100000000000000L) return 15;
              if (l >= 10000000000000L) return 14;
              if (l >= 1000000000000L) return 13;
              if (l >= 100000000000L) return 12;
              if (l >= 10000000000L) return 11;
              if (l >= 1000000000L) return 10;
              if (l >= 100000000L) return 9;
              if (l >= 10000000L) return 8;
              if (l >= 1000000L) return 7;
              if (l >= 100000L) return 6;
              if (l >= 10000L) return 5;
              if (l >= 1000L) return 4;
              if (l >= 100L) return 3;
              if (l >= 10L) return 2;
              return 1;
          }
  
          private static int countDigits(int l) {
              if (l >= 1000000000) return 10;
              if (l >= 100000000) return 9;
              if (l >= 10000000) return 8;
              if (l >= 1000000) return 7;
              if (l >= 100000) return 6;
              if (l >= 10000) return 5;
              if (l >= 1000) return 4;
              if (l >= 100) return 3;
              if (l >= 10) return 2;
              return 1;
          }
  
      }
  
  
      static class FastInput {
          private final StringBuilder defaultStringBuf = new StringBuilder (1 << 13);
          private final ByteBuffer tokenBuf = new ByteBuffer ();
          private final byte[] buf = new byte[1 << 13];
          private SpaceCharFilter filter;
          private final InputStream is;
          private int bufOffset;
          private int bufLen;
          private int next;
          private int ptr;
  
          public FastInput(InputStream is) {
              this.is = is;
          }
  
          private int read() {
              while (bufLen == bufOffset) {
                  bufOffset = 0;
                  try {
                      bufLen = is.read (buf);
                  } catch (IOException e) {
                      bufLen = -1;
                  }
                  if (bufLen == -1) {
                      return -1;
                  }
              }
              return buf[bufOffset++];
          }
  
          public void skipBlank() {
              while (next >= 0 && next <= 32) {
                  next = read ();
              }
          }
  
          public String next() {
              return readString ();
          }
  
          public int ri() {
              return readInt ();
          }
  
          public int readInt() {
              boolean rev = false;
  
              skipBlank ();
              if (next == '+' || next == '-') {
                  rev = next == '-';
                  next = read ();
              }
  
              int val = 0;
              while (next >= '0' && next <= '9') {
                  val = val * 10 - next + '0';
                  next = read ();
              }
  
              return rev ? val : -val;
          }
  
          public long readLong() {
              boolean rev = false;
  
              skipBlank ();
              if (next == '+' || next == '-') {
                  rev = next == '-';
                  next = read ();
              }
  
              long val = 0L;
              while (next >= '0' && next <= '9') {
                  val = val * 10 - next + '0';
                  next = read ();
              }
  
              return rev ? val : -val;
          }
  
          public long rl() {
              return readLong ();
          }
  
          public String readString(StringBuilder builder) {
              skipBlank ();
  
              while (next > 32) {
                  builder.append ((char) next);
                  next = read ();
              }
  
              return builder.toString ();
          }
  
          public String readString() {
              defaultStringBuf.setLength (0);
              return readString (defaultStringBuf);
          }
  
          public int rs(char[] data, int offset) {
              return readString (data, offset);
          }
  
          public char[] rsc() {
              return readString ().toCharArray ();
          }
  
          public int rs(char[] data) {
              return rs (data, 0);
          }
  
          public int readString(char[] data, int offset) {
              skipBlank ();
  
              int originalOffset = offset;
              while (next > 32) {
                  data[offset++] = (char) next;
                  next = read ();
              }
  
              return offset - originalOffset;
          }
  
          public char rc() {
              return readChar ();
          }
  
          public char readChar() {
              skipBlank ();
              char c = (char) next;
              next = read ();
              return c;
          }
  
          public double rd() {
              return nextDouble ();
          }
  
          public double nextDouble() {
              int c = read ();
              while (isSpaceChar (c))
                  c = read ();
              int sgn = 1;
              if (c == '-') {
                  sgn = -1;
                  c = read ();
              }
              double res = 0;
              while (!isSpaceChar (c) && c != '.') {
                  if (c == 'e' || c == 'E')
                      return res * Math.pow (10, readInt ());
                  if (c < '0' || c > '9')
                      throw new InputMismatchException ();
                  res *= 10;
                  res += c - '0';
                  c = read ();
              }
              if (c == '.') {
                  c = read ();
                  double m = 1;
                  while (!isSpaceChar (c)) {
                      if (c == 'e' || c == 'E')
                          return res * Math.pow (10, readInt ());
                      if (c < '0' || c > '9')
                          throw new InputMismatchException ();
                      m /= 10;
                      res += (c - '0') * m;
                      c = read ();
                  }
              }
              return res * sgn;
          }
  
          public boolean isSpaceChar(int c) {
              if (filter != null)
                  return filter.isSpaceChar (c);
              return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
          }
  
          public interface SpaceCharFilter {
              public boolean isSpaceChar(int ch);
          }
  
          public final int readByteUnsafe() {
              if (ptr < bufLen) return buf[ptr++];
              ptr = 0;
              try {
                  bufLen = is.read (buf);
                  if (bufLen > 0) {
                      return buf[ptr++];
                  } else {
                      return -1;
                  }
              } catch (IOException e) {
                  throw new UncheckedIOException (e);
              }
          }
  
          public final int readByte() {
              if (ptr < bufLen) return buf[ptr++];
              ptr = 0;
              try {
                  bufLen = is.read (buf);
                  if (bufLen > 0) {
                      return buf[ptr++];
                  } else {
                      throw new java.io.EOFException ();
                  }
              } catch (IOException e) {
                  throw new UncheckedIOException (e);
              }
          }
  
          public final String nextLine() {
              tokenBuf.clear ();
              for (int b = readByte (); b != '\n'; b = readByteUnsafe ()) {
                  if (b == -1) break;
                  tokenBuf.append (b);
              }
              return new String (tokenBuf.getRawBuf (), 0, tokenBuf.size ());
          }
  
          public final String nl() {
              return nextLine ();
          }
  
          public final boolean hasNext() {
              for (int b = readByteUnsafe (); b <= 32 || b >= 127; b = readByteUnsafe ()) {
                  if (b == -1) return false;
              }
              --ptr;
              return true;
          }
  
          public void readArray(Object T) {
              if (T instanceof int[]) {
                  int[] arr = (int[]) T;
                  for (int i = 0; i < arr.length; i++) {
                      arr[i] = ri ();
                  }
              }
              if (T instanceof long[]) {
                  long[] arr = (long[]) T;
                  for (int i = 0; i < arr.length; i++) {
                      arr[i] = rl ();
                  }
              }
              if (T instanceof double[]) {
                  double[] arr = (double[]) T;
                  for (int i = 0; i < arr.length; i++) {
                      arr[i] = rd ();
                  }
              }
              if (T instanceof char[]) {
                  char[] arr = (char[]) T;
                  for (int i = 0; i < arr.length; i++) {
                      arr[i] = readChar ();
                  }
              }
              if (T instanceof String[]) {
                  String[] arr = (String[]) T;
                  for (int i = 0; i < arr.length; i++) {
                      arr[i] = next ();
                  }
              }
              if (T instanceof int[][]) {
                  int[][] arr = (int[][]) T;
                  for (int i = 0; i < arr.length; i++) {
                      for (int j = 0; j < arr[0].length; j++) {
                          arr[i][j] = ri ();
                      }
                  }
              }
              if (T instanceof char[][]) {
                  char[][] arr = (char[][]) T;
                  for (int i = 0; i < arr.length; i++) {
                      for (int j = 0; j < arr[0].length; j++) {
                          arr[i][j] = readChar ();
                      }
                  }
              }
              if (T instanceof long[][]) {
                  long[][] arr = (long[][]) T;
                  for (int i = 0; i < arr.length; i++) {
                      for (int j = 0; j < arr[0].length; j++) {
                          arr[i][j] = rl ();
                      }
                  }
  
              }
          }
  
          public final void close() {
              try {
                  is.close ();
              } catch (IOException e) {
                  throw new UncheckedIOException (e);
              }
          }
  
  
          private static final class ByteBuffer {
              private static final int DEFAULT_BUF_SIZE = 1 << 12;
              private byte[] buf;
              private int ptr = 0;
  
              private ByteBuffer(int capacity) {
                  this.buf = new byte[capacity];
              }
  
              private ByteBuffer() {
                  this (DEFAULT_BUF_SIZE);
              }
  
              private ByteBuffer append(int b) {
                  if (ptr == buf.length) {
                      int newLength = buf.length << 1;
                      byte[] newBuf = new byte[newLength];
                      System.arraycopy (buf, 0, newBuf, 0, buf.length);
                      buf = newBuf;
                  }
                  buf[ptr++] = (byte) b;
                  return this;
              }
  
              private char[] toCharArray() {
                  char[] chs = new char[ptr];
                  for (int i = 0; i < ptr; i++) {
                      chs[i] = (char) buf[i];
                  }
                  return chs;
              }
  
              private byte[] getRawBuf() {
                  return buf;
              }
  
              private int size() {
                  return ptr;
              }
  
              private void clear() {
                  ptr = 0;
              }
          }
  
  
      }
  
  
      static class Debug {
          private final boolean offline;
          private final PrintStream out = System.err;
          static int[] empty = new int[0];
  
          public Debug(boolean enable) {
              offline = enable && System.getSecurityManager () == null;
          }
  
          public Debug debug(String name, Object x) {
              return debug (name, x, empty);
          }
  
          public Debug debug(String name, long x) {
              if (offline) {
                  debug (name, "" + x);
              }
              return this;
          }
  
          public Debug debug(String name, String x) {
              if (offline) {
                  out.printf ("%s=%s", name, x);
                  out.println ();
              }
              return this;
          }
  
          public Debug debug(String name, Object x, int... indexes) {
              if (offline) {
                  if (x == null || !x.getClass ().isArray ()) {
                      out.append (name);
                      for (int i : indexes) {
                          out.printf ("[%d]", i);
                      }
                      out.append ("=").append ("" + x);
                      out.println ();
                  } else {
                      indexes = Arrays.copyOf (indexes, indexes.length + 1);
                      if (x instanceof byte[]) {
                          byte[] arr = (byte[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      } else if (x instanceof short[]) {
                          short[] arr = (short[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      } else if (x instanceof boolean[]) {
                          boolean[] arr = (boolean[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      } else if (x instanceof char[]) {
                          char[] arr = (char[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      } else if (x instanceof int[]) {
                          int[] arr = (int[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      } else if (x instanceof float[]) {
                          float[] arr = (float[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      } else if (x instanceof double[]) {
                          double[] arr = (double[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      } else if (x instanceof long[]) {
                          long[] arr = (long[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      } else {
                          Object[] arr = (Object[]) x;
                          for (int i = 0; i < arr.length; i++) {
                              indexes[indexes.length - 1] = i;
                              debug (name, arr[i], indexes);
                          }
                      }
                  }
              }
              return this;
          }
  
      }
  }