#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
#define drep(i, cc, n) for (ll i = (cc); i <= (n); ++i)
#define rep(i, n) drep(i, 0, n - 1)
#define all(a) (a).begin(), (a).end()
#define pb push_back
#define fi first
#define se second

const ll MOD = 1000000007;
const ll MOD2 = 998244353;
const ll INF = 1LL << 60;
const ll N_MAX = 2e5;

int main(){
    ll n;
    string s;
    cin >> n >> s;

    ll sum = 0;
    rep(i, n) if(s[i]=='A' || s[i]=='G' || s[i]=='C' || s[i]=='T') sum++;
    set<ll> st;
    rep(i, n) st.insert(i);

    auto now = st.lower_bound(sum-1);
    ll ans = 0;
    while(sum != 0){
        ans++;
        if(s[*now] == 'A' || s[*now] == 'G' || s[*now] == 'C' || s[*now] == 'T'){
            sum--;
            auto tmp = now;
            st.erase(now--);
        }else{
            st.erase(now++);
        }
    }

    cout << ans << endl;
}