#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

string s, rs;
int n;
vector<int> p;

void preffix_function(string s, vector<int>& p){
    int n = s.length();
    p.assign(n,0);
    for(int i = 1; i < n; ++i){
        for(int j = p[i-1]; ;j = p[j-1]){
            if(s[i]==s[j]){ p[i] = j+1; break;}
            if(!j) break;
        }
    }
}

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cin >> n >> s;
    rs = s;
    reverse(rs.begin(),rs.end());
    s = rs+"#"+s+s;
    preffix_function(s,p);
    if(*max_element(p.begin(),p.end())==n) cout << max_element(p.begin(),p.end())-p.begin()-2*n;
    else cout << -1;


    return 0;
    }