#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
#define ALL(obj) (obj).begin(), (obj).end()
#define rALL(obj) (obj).rbegin(), (obj).rend()
using ll = long long;

const int INF = 1 << 30;

int main() {
  int N;
  string S;
  cin >> N >> S;

  int ans = 0;
  for (int i = 0; i < N; i++) {
    if(S[i] == 'A' || S[i] == 'G' || S[i] == 'C' || S[i] == 'T') ans = i+1;
  }

  cout << ans << endl;
  return 0;
}