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

int main()
{
	int N;
	string S;
	cin >> N >> S;
	
	int cnt = 0;
	for(int i = 0; i < N; i++) if(S[i] == 'A' || S[i] == 'G' || S[i] == 'C' || S[i] == 'T') cnt++;
	int ans = 0;
	while(cnt != 0)
	{
		for(int i = cnt - 1; ; i++)
		{
			ans++;
			if(S[i] == 'A' || S[i] == 'G' || S[i] == 'C' || S[i] == 'T')
			{
				S[i] = 'X';
				break;
			}
		}
		cnt--;
	}
	cout << ans << endl;
	return 0;
}