#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000

bool check(char c){
	if(c=='A'||c=='G'||c=='T'||c=='C')return true;
	return false;
}


int main() {
    
	int n;
	cin>>n;
	string s;
	cin>>s;
	
	int c = 0;
	rep(i,n){
		if(check(s[i]))c++;
	}
	
	deque<char> X,Y;
	rep(i,n)X.push_back(s[i]);
	
	while(c!=0){
		while(X.size()>c){
			Y.push_front(X.back());
			X.pop_back();
		}
		while(X.size()<c){
			X.push_back(Y.front());
			Y.pop_front();
		}
		if(check(X.back()))c--;
		X.pop_back();
		
	}
	n -= X.size();
	n -= Y.size();
	cout<<n<<endl;
    return 0;
}