#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;

int N, pos = 0;
string S;

int main() {
  cin >> N >> S;
  for (int i = 0; i < N; i++) {
    if (S[i] == 'A' || S[i] == 'G' || S[i] == 'C' || S[i] == 'T') pos = i + 1;
  }
  cout << pos << endl;
  return 0;
}