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: from __future__ import division, print_function
  
  import os,sys
  from io import BytesIO, IOBase
  
  if sys.version_info[0] < 3:
      from __builtin__ import xrange as range
      from future_builtins import ascii, filter, hex, map, oct, zip
  
  from bisect import bisect_left as lower_bound, bisect_right as upper_bound 
  def so():  return int(input())
  def st():  return input()
  def mj():  return map(int,input().strip().split(" "))
  def msj(): return map(str,input().strip().split(" "))
  def le():  return list(map(int,input().split()))
  def rc():  return map(float,input().split())
  def lebe():return list(map(int, input()))
  
  def dmain():
      sys.setrecursionlimit(1000000)
      threading.stack_size(1024000)
      thread = threading.Thread(target=main)
      thread.start()
  def joro(L):
      return(''.join(map(str, L)))
  
  
  def decimalToBinary(n): return bin(n).replace("0b","")
  
  
  def isprime(n):
      for i in range(2,int(n**0.5)+1):
          if n%i==0:
              return False
      return True
  def npr(n, r):
      return factorial(n) // factorial(n - r) if n >= r else 0
   
   
  def ncr(n, r):
      return factorial(n) // (factorial(r) * factorial(n - r)) if n >= r else 0
   
   
  def lower_bound(li, num):
      answer = -1
      start = 0
      end = len(li) - 1
   
      while (start <= end):
          middle = (end + start) // 2
          if li[middle] >= num:
              answer = middle
              end = middle - 1
          else:
              start = middle + 1
      return answer  # min index where x is not less than num
   
   
  def upper_bound(li, num):
      answer = -1
      start = 0
      end = len(li) - 1
   
      while (start <= end):
          middle = (end + start) // 2
   
          if li[middle] <= num:
              answer = middle
              start = middle + 1
   
          else:
              end = middle - 1
      return answer  # max index where x is not greater than num
  def tir(a,b,c):
      if(0==c):
          return 1
      if(len(a)<=b):
          return 0
      
      if(c!=-1):
          return (tir(a,1+b,c+a[b]) or tir(a,b+1,c-a[b]) or tir(a,1+b,c)) 
          
          
      else:
          return (tir(a,1+b,a[b]) or tir(a,b+1,-a[b]) or tir(a,1+b,-1))
  hoi=int(2**20)        
   
  def abs(x):
      return x if x >= 0 else -x
   
   
  def binary_search(li, val, lb, ub):
      # print(lb, ub, li)
      ans = -1
      while (lb <= ub):
          mid = (lb + ub) // 2
          # print('mid is',mid, li[mid])
          if li[mid] > val:
              ub = mid - 1
          elif val > li[mid]:
              lb = mid + 1
          else:
              ans = mid  # return index
              break
      return ans
   
   
  def kadane(x):  # maximum sum contiguous subarray
      sum_so_far = 0
      current_sum = 0
      for i in x:
          current_sum += i
          if current_sum < 0:
              current_sum = 0
          else:
              sum_so_far = max(sum_so_far, current_sum)
      return sum_so_far
   
   
  def pref(li):
      pref_sum = [0]
      for i in li:
          pref_sum.append(pref_sum[-1] + i)
      return pref_sum
   
   
  def SieveOfEratosthenes(n):
      prime = [True for i in range(n + 1)]
      p = 2
      li = []
      while (p * p <= n):
          if (prime[p] == True):
              for i in range(p * p, n + 1, p):
                  prime[i] = False
          p += 1
   
      for p in range(2, len(prime)):
          if prime[p]:
              li.append(p)
      return li
   
   
  def primefactors(n):
      factors = []
      while (n % 2 == 0):
          factors.append(2)
          n //= 2
      for i in range(3, int(sqrt(n)) + 1, 2):  # only odd factors left
          while n % i == 0:
              factors.append(i)
              n //= i
      if n > 2:  # incase of prime
          factors.append(n)
      return factors
           
      
  def read():
      sys.stdin  = open('input.txt', 'r')  
      sys.stdout = open('output.txt', 'w') 
  def tr(n):
      return n*(n+1)//2
  boi=int(1e9-7)
  doi=int(1e9+7)
  uoi=int(1+1e6)
  INT_MAX = 99999999999
   
  def ft(s,c):
      l=0
      r=len(s)-1
      d=0
      while(r>l):
          if(s[l]!=s[r]):
              f=0
              if(s[l]==c):
                  l+=1
                  f=1
              elif(s[r]==c):
                  r-=1
                  f=1
              d=1+d
              if(f==0):
                 return -1
          else:
              l+=1
              r-=1
      return d
      
      
  
          
      
  def iu():
      import sys
      import math as my
      input=sys.stdin.readline
      m=so()
      b=st()
      er=0
      br=sys.maxsize
      
      for i in range(97,123):
          
          s=0
          e=m-1
          d=1
          u=0
          while(s<e):
              if(b[s]==b[e]):
                  s+=1
                  e-=1
              elif(b[s]==chr(i)):
                  s+=1
                  u+=1
              else:
                  if(b[e]==chr(i)):
                      e-=1
                      u+=1
                  else:
                      d=0
                      break
          
          if(d==1):
              er=1
              br=min(br,u)
      if(er==1):
          print(br)
      else:
          print(-1)
          
          
          
      
  def main():
      for i in range(so()):
          iu()
          
      
      
      
      
      
      
      
      
      
      
  
      
  
      
          
          
              
      
      
                  
              
                  
                  
                  
                  
              
      
              
              
      
      
      
      
          
              
      
             
              
              
          
      
          
      
      
          
          
          
  
              
                      
                  
                      
                      
                      
                      
                      
                      
          
                  
          
          
         
      
             
            
            
                  
              
          
                  
      
          
          
          
         
              
                  
          
  
  
  
  
  # region fastio
  # template taken from https://github.com/cheran-senthil/PyRival/blob/master/templates/template.py
  
  BUFSIZE = 8192
  
  
  class FastIO(IOBase):
      newlines = 0
  
      def __init__(self, file):
          self._fd = file.fileno()
          self.buffer = BytesIO()
          self.writable = "x" in file.mode or "r" not in file.mode
          self.write = self.buffer.write if self.writable else None
  
      def read(self):
          while True:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              if not b:
                  break
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines = 0
          return self.buffer.read()
  
      def readline(self):
          while self.newlines == 0:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              self.newlines = b.count(b"\n") + (not b)
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines -= 1
          return self.buffer.readline()
  
      def flush(self):
          if self.writable:
              os.write(self._fd, self.buffer.getvalue())
              self.buffer.truncate(0), self.buffer.seek(0)
  
  
  class IOWrapper(IOBase):
      def __init__(self, file):
          self.buffer = FastIO(file)
          self.flush = self.buffer.flush
          self.writable = self.buffer.writable
          self.write = lambda s: self.buffer.write(s.encode("ascii"))
          self.read = lambda: self.buffer.read().decode("ascii")
          self.readline = lambda: self.buffer.readline().decode("ascii")
  
  
  def print(*args, **kwargs):
      """Prints the values to a stream, or to sys.stdout by default."""
      sep, file = kwargs.pop("sep", " "), kwargs.pop("file", sys.stdout)
      at_start = True
      for x in args:
          if not at_start:
              file.write(sep)
          file.write(str(x))
          at_start = False
      file.write(kwargs.pop("end", "\n"))
      if kwargs.pop("flush", False):
          file.flush()
  
  
  if sys.version_info[0] < 3:
      sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
  else:
      sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
  
  input = lambda: sys.stdin.readline().rstrip("\r\n")
  
  # endregion
  
  
  if __name__ == "__main__":
      #read()
      main()
      #dmain()
  
  # Comment Read()