#include <bits/stdc++.h>
#include<iostream>
#include<map>
#include<vector>
#include <algorithm>
#include<math.h>
#include <iomanip>
#include<set>
#include <numeric>
#include<string>
using ll = long long;
using namespace std;

int main()
{
    ll n, cnt = 0, ans = 0;
    string s;
    cin >> n >> s;
    for (ll i = 0; i < n; i++) {
        if (s[i] == 'A' || s[i] == 'T' ||s[i] == 'G' ||s[i] == 'C'){
            cnt++;
            ans = i + 1;
        }
    }
    cout << ans << endl;
}