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

using P = pair<int, int>;
#define test cout << "test:"
const int inf = 1073741824;
const long long linf = 1152921504606846976;
// const int mod = 998244353;
// const int mod = 1000000007;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};

void _main()
{
	int n;
	string s;
	cin >> n >> s;

	int cnt = 0;
	for (int i = 0; i < n; i++)
		if (s[i] == 'A' or s[i] == 'G' or s[i] == 'C' or s[i] == 'T')
			cnt = i + 1;

	cout << cnt << '\n';
	return;
}

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout << fixed << setprecision(15);
	_main();
}