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

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

	set<int> st;
	for( int i = 0; i < n; i++ ) {
		if( s[i] == 'A' || s[i] == 'G' || s[i] == 'C' || s[i] == 'T' ) st.insert( i );
	}
	int ans = 0;
	int last = -1;
	int del = 0;
	while( st.size() ) {
		int j = st.size() - 1;
		auto it = st.lower_bound( j );
		int a = *it - j + 1;
		if( last < 0 ) {
			last = *it;
			del += a;
		}
		else if( *it > last ) {
			int t = a;
			a -= del;
			del = t;
			last = *it;
		}
		else {
			last = *it;
			del += a;
		}
		ans += a;
		st.erase( it );
	}

	cout << ans << endl;
}