#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <fstream>
#include <cmath>
#include <stack>
#include <queue>
#include <iomanip>
using namespace std;

int main(){
  int n;cin>>n;
  string s;cin>>s;

  int index = 0;
  for(int i=1;i<=n;i++){
    if(s[i-1]=='A' || s[i-1]=='G' || s[i-1]=='C' || s[i-1]=='T') index = i;
  }
  cout << index << endl;
}