typedef int ll;
typedef long double ld;
#include <bits/stdc++.h>
using namespace std;
// #define int long long

signed main(){
    ll n;
    std::cin >> n;
    string s;
    std::cin >> s;
    ll cnt = 0;
    for (auto e : s) {
        if(e=='A'||e=='G'||e=='C'||e=='T'){
            cnt++;
        }
    }
    ll ans= 0;
    if(cnt==0){
        std::cout << 0 << std::endl;
        return 0;
    }
    
    ll l,r;
    ll cur = cnt;
    l = r = cur-1;
    bool isr = true;
    while(cnt>0){
        ans++;
        // std::cout << l<<" "<<r<<" "<<cnt << std::endl;
        if(l==r){
            cur = r;
        }else if(isr){
            cur = r;
            r++;
        }else{
            cur = l;
            l--;
        }
        if(s[cur]=='A'||s[cur]=='G'||s[cur]=='C'||s[cur]=='T'){
            isr = false;
            cnt--;
        }else{
            isr = true;
        }
        if(l==r){
            l--;
            r++;
        }
    }
    std::cout << ans << std::endl;
}