#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <map>
#include <set>
#include <queue>
#include <iomanip>
#include <cstring>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long ll;
#define rep(i,n) for (int i = 0; i < int(n);i++)

int main(){
  int n;
  string s;
  cin >> n >> s;
  int id = -1;
  for (int i = 0; i < n;i++){
    if (s[i] == 'A' || s[i] == 'T' || s[i] == 'G' || s[i] == 'C') id = i;
  }
  cout << s.substr(0,id+1).size() << endl;
  return 0;
}