#include <bits/stdc++.h>
using namespace std ;
#define fast_input_output ios::sync_with_stdio(false); cin.tie(nullptr);
typedef long long ll ;
typedef long double ld ;
typedef pair<ll,ll> P ;
typedef tuple<ll,ll,ll> TP ;
#define chmin(a,b) a = min(a,b)
#define chmax(a,b) a = max(a,b)
#define bit_count(x) __builtin_popcountll(x)
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) a / gcd(a,b) * b
#define rep(i,n) for(int i = 0 ; i < n ; i++)
#define rrep(i,a,b) for(int i = a ; i < b ; i++)
#define endl "\n"

int n ;
string S ;
set<int> st ;

int main(){
    // fast_input_output
    cin >> n >> S ;
    rep(i,n) st.insert(i) ;
    int sum = 0 ;
    for(char u : S){
        if(u == 'A') sum++ ;
        if(u == 'G') sum++ ;
        if(u == 'C') sum++ ;
        if(u == 'T') sum++ ;
    }
    while(sum > 0){
        auto it = st.lower_bound(sum-1) ;
        st.erase(it) ;
        if(S[*it] == 'A' || S[*it] == 'G' || S[*it] == 'C' || S[*it] == 'T') sum-- ;
    }
    cout << n - (int)st.size() << endl ;
}