#pragma GCC omptimize("unroll-loops")
    #pragma GCC omptimize("игорь говно")
    #pragma GCC optimize("no-stack-protector")
    #pragma comment(linker, "/STACK:1000000000")


#include <iostream>
#include <cmath>
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <set>
#include <map>
#include <list>
#include <time.h>
#include <math.h>
#include <random>
#include <deque>
#include <queue>
#include <cassert>
#include <unordered_map>
#include <unordered_set>
#include <iomanip>
#include <bitset>
#include <sstream>
#include <chrono>
    #define LOCAL
    #define fori(i, r, x) for(int i = r; i < x; i++)
    #define eb emplace_back
    #define lb lower_bound
    #define ub upper_bound
    #define er equal_range
    #define fi first
    #define se second

    using namespace std;

    typedef long long int lli;
    typedef long long ll;
    typedef long double ld;
    typedef pair<int, int> pii;
    typedef pair<ll,ll> pll;
    typedef pair<ld,ld> pld;
    typedef vector<int> vi;
    typedef vector<ll> vll;
    typedef vector<ld> vld;
    typedef vector<vector<int>> vvi;

    const int INF32 = 2009000999;
    const ll INF64 = 4e18;
    const int sz = 1e6;
    const int mod = 1e9+7;
    const long double EPS = 1e-18;
    const int base = 1000*1000*1000;
    const int p = 31;


    string s;
    ll cnt = 0, n;

    ll haash(string a, int x) {
        long long hashh = 0, p_pow = 1;
        for (int j=x; j<a.size(); j++)  {
            hashh += (a[j] - 'a' + 1) * p_pow;
            p_pow *= p;
        }
        for (int j=0; j<x; j++)  {
            hashh += (a[j] - 'a' + 1) * p_pow;
            p_pow *= p;
        }
        return hashh;
    }

    ll b, c;
   inline void r() {
        cin >> n;
        cin >> s;
        string s1(s);
        reverse(s1.begin(), s1.end());

        b = haash(s, 0);
       // cerr << b << endl;
        c = haash(s1, 0);
       // for(int i = 0; i < s.size() - 1; i++) step();

       // cerr << c << endl;
      //  cerr << haash(s) << endl;
        if (b == c) {
            cout << "0";
            return;
        }
        ll k = 0;
        cnt = 0;
        ll ans = 0;
        ll cur;
        for(int i = 0; i < s.size(); i++) {
            //step();
           // cerr << s << endl;
            cur = haash(s,i);
           // cerr << cur << endl;
            if (cur == c) {
                ans = cnt + 1;
                cout << ans;
                return;
            }
            else if ((cur == b) && (i != 0)){
                cout << -1;
                return;
            }
            else if (i != 0){
                cnt++;
            }
        }
    }

    int main(){
        ios_base::sync_with_stdio(false);
        cin.tie(0);
        cout.tie(0);

        #ifndef LOCAL
            freopen("input.txt", "r", stdin);
            freopen("output.txt", "w", stdout);
        #endif
       r();
        return 0;

    }