https://codeforces.com/contest/1582/problem/C
#include<iostream>
#include<algorithm>
#include<bits/stdc++.h>
using namespace std;

const int N = 1e3 + 10;
typedef long long ll;


int f[N][N];
 
 
 void solve() {
  int n; 
  string s;
  cin >> n >> s;
  int ans = n + 1;
   for (int c = 0; c < 26; c ++) {
    int l = 0, r = n - 1;
    int cnt = 0;
     while(l <= r) {
      if (s[l] == s[r]) {
        l ++, r --;
       }
       else if (s[l] == char(c + 'a')) {
        l ++;
        cnt ++;
       }
       else if (s[r] == char(c + 'a')) {
        r --;
        cnt ++;
       }
       else {
        cnt = n + 1;
        break;
       }
     }
     ans = min(ans, cnt);
   }
   if (ans == n + 1) ans = - 1;
  cout << ans << endl;
 }
int main () {
  int t;
  cin >> t;
  while (t --) solve();
  return 0;
}