#include <iostream>
using namespace std;
int main(){
    int n;cin>>n;
    string s;cin>>s;
    int ans = 0;
    for(int i = 0; s.size() > i; i++){
        if(s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T'){
            ans = i+1;
        }
    }
    cout << ans << endl;
}