#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>

#define rep(i,n) for(i=0; i<n; ++i)
#define in(a) cin >> a
#define out(a,b) cout << a << b
using namespace std;
using lint = long long;

int main(void){
    int i, j, k, v, y, n, m=0, cnt=0, c=0;
    string s, t="AGCT";
    in(n);
    in(s);
    vector<bool> x(n,true);
    rep(i,n){
        rep(j,4){
            c+=s[i]==t[j];
        }
    }
    while(c){
        m=0;
        rep(i,n){
            if(x[i]){
                if(m==c-1)  {
                    x[i]=false;
                    cnt++;
                    rep(j,4){
                        c-=s[i]==t[j];
                    }
                    break;
                }
                m++;
            }
        }
    }
    out(cnt,endl);

    return 0;
}