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

#define rep(i,n) for(i=0; i<n; ++i)
#define in(a) cin >> a
#define out(a,b) cout << a << b
using namespace std;
using lint = long long;

int main(void){
    int i, j, n, m, k, cnt = 0;
    string s, u="AGCT";

    in(n);
    in(s);
    rep(i,n){
        rep(j,4){
            if(s[i]==u[j])  cnt=i+1;
        }

    }    
    out(cnt,endl);

    return 0;
}