#include <bits/stdc++.h>
using namespace std;

int n, m, c, t;
string s;

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

	for (int i = 0; i < s.size(); i++) {
		if(s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T')
			m = i + 1;
	}

	cout << m << endl;
}