#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vll = vector<ll>;
#define all(A) A.begin(),A.end()
#define rep(i, n) for (ll i = 0; i < (ll) (n); i++)



int main() {
    
        ll N;
        cin>>N;
        string S;
        cin>>S;
        string T="AGCT";
        ll an=0;
        rep(i,N){
            rep(j,4){
                if(S[i]==T[j])an=i+1;
            }
        }
        cout<<an<<endl;
    

    
}