#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define MOD1 1000000007
#define MOD2 998244353
#define NO cout << "NO" << endl
#define YES cout << "YES" << endl
ll expo(ll a, ll b, ll mod) {ll res = 1; while (b > 0) {if (b & 1)res = (res * a) % mod; a = (a * a) % mod; b = b >> 1;} return res;}
ll mminvprime(ll a, ll b) {return expo(a, b - 2, b);}
ll mod_add(ll a, ll b, ll m) {a = a % m; b = b % m; return (((a + b) % m) + m) % m;}
ll mod_mul(ll a, ll b, ll m) {a = a % m; b = b % m; return (((a * b) % m) + m) % m;}
ll mod_sub(ll a, ll b, ll m) {a = a % m; b = b % m; return (((a - b) % m) + m) % m;}
ll mod_div(ll a, ll b, ll m) {a = a % m; b = b % m; return (mod_mul(a, mminvprime(b, m), m) + m) % m;}
void print(vector<int> &ans){for(auto x: ans) cout<<x<<" "; cout<<endl;}

// ========================================================================


void solve()
{

    int n;string s;
    cin>>n;
    cin>>s;
    if(n%2!=0)
    {
        cout<<-1<<endl;
        return;
    }
    vector<int> hm(26,0);
    vector<int> rep(26,0);

    for(int i=0;i<n;i++)
    {
        hm[s[i]-'a']+=1;

        if(hm[s[i]-'a']>n/2)
        {
            cout<<-1<<endl;
            return;
        }
    }

    for(int i=0;i<int(n/2);i++)
    {
        if(s[i] == s[n-i-1])
            rep[s[i]-'a']+=1;
    }

    vector<int> _rep;
    for(int i=0;i<26;i++)
    {
        if(rep[i]!=0)
            _rep.push_back(rep[i]);
    }

    if(_rep.size()==0)
    {
        cout<<0<<endl;
        return;
    }

    if(_rep.size()==1)
    {
        cout<<_rep[0]<<endl;
        return;
    }
    sort(_rep.begin(),_rep.end());
    int n1=_rep.size();
    vector<int> prefix(n1,0);
    vector<int> suffix(n1,0);

    prefix[0]=_rep[0];
    suffix[n1-1]=_rep[n1-1];

    for(int i=1;i<n1;i++)
        prefix[i]=prefix[i-1]+_rep[i];

    for(int i=n1-2;i>=0;i--)
        suffix[i]=suffix[i+1]+_rep[i];









    int ind=0;

    for(int i=n1-1;i>0;i--)
    {

        if(suffix[i] == prefix[i-1])
        {
            cout<<suffix[i]<<endl;
            return;
        }
        else if(suffix[i] > prefix[i-1])
        {
            ind=i;
            break;
        }
    }


    cout<<prefix[ind-1]+int(ceil((1.0*suffix[ind]-prefix[ind-1])/2))<<endl;






}


int main()
{
    int T;cin>>T;
    while(T--)
    {
        solve();
    }
}