#include <bits/stdc++.h>
#include <atcoder/all>

typedef unsigned long long ULLONG;
typedef long long LLONG;
static const LLONG MOD_NUM = 998244353LL;//1000000007LL;
static const LLONG INF_NUM = 1e18;
static const int CH_KINDS = 'z' - 'a' + 1;
template<class _T> static void get(_T& a) {
	std::cin >> a;
}
template<class _T> static void get(_T& a, _T& b) {
	std::cin >> a >> b;
}
template<class _T> static void get(_T& a, _T& b, _T& c) {
	std::cin >> a >> b >> c;
}
template <class _T> static _T tp_abs(_T a) {
	if (a < (_T)0) {
		a *= (_T)-1;
	}
	return a;
}
template <class _T> static _T tp_pow(int x, int exp)
{
	_T ans = 1;
	_T base = x;
	while (exp > 0) {
		if (exp & 1) {
			ans = (ans * base);
		}
		base = (base * base);
		exp >>= 1;
	}
	return ans;
}

static void task();

int main()
{
	task();
	fflush(stdout);
	return 0;
}

static void task()
{
	int N;
	get(N);

	std::string si;
	get(si);

	int c = 0;
	for (int i = 0; i < N; i++) {
		if (si[i] == 'A' || si[i] == 'G' || si[i] == 'C' || si[i] == 'T') c++;
	}
	std::deque<char> fr, rr;
	for (int i = 0; i < N; i++) {
		if (fr.size() < c) fr.push_back(si[i]);
		else rr.push_back(si[i]);
	}
	int ans = 0;
	while (c) {
		int tar = 0;
		if (fr.size() == c) {
			tar = fr.back(); fr.pop_back();
		}
		else {
			tar = rr.front(); rr.pop_front();
		}
		ans++;
		if (tar == 'A' || tar == 'G' || tar == 'C' || tar == 'T') c--;
	}
	printf("%d\n", ans);
}