#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 1000000001

char get(char c,int a){
	a += 26;
	a %= 26;
	rep(i,a){
		c++;
		if(c>'Z')c = 'A';
	}
	return c;
}

int main() {
    
	int n;
	cin>>n;
	string s;
	cin>>s;
	
	fenwick_tree<int> F(n);
	rep(i,n)F.add(i,1);
	
	vector<int> c(26);
	rep(i,n){
		c[s[i]-'A']++;
	}
	
	int add = 0;
	
	while(true){
		int c0 = 0;
		c0 += c[get('A',-add)-'A'];
		c0 += c[get('G',-add)-'A'];
		c0 += c[get('C',-add)-'A'];
		c0 += c[get('T',-add)-'A'];
		if(c0==0)break;
		int ok = 0,ng = n;
		while(ng-ok>1){
			int mid = (ok+ng)/2;
			if(F.sum(0,mid)<c0)ok = mid;
			else ng = mid;
		}
		add += c[s[ok]-'A']-1;
		c[s[ok]-'A']--;
		s[ok] = '-';
		F.add(ok,-1);
	}
	int ans = 0;
	rep(i,s.size()){
		if(s[i]!='-'){
			//ans += get(s[i],add);
		}
		else ans++;
	}
	cout<<ans<<endl;
	
    return 0;
}