#include <iostream>
#include <vector>
#include <string>

#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, n, m, cnt = 0, sum = 1;
    in(n);
    string s,t="AGCT";
    in(s);
    while(sum){
        sum=0;
        k=s.size();
        rep(i,k){
            rep(j,4){
                sum+=s[i]==t[j];
            }
            //out(sum,endl);
        }
        
        if(sum!=0) {s.erase(sum-1,1);
        cnt++;}
        else break;
    }
    out(cnt,endl);
    return 0;
}