#include <bits/stdc++.h>
#define be(v) (v).begin(),(v).end()
#define pb(q) push_back(q)
#define rep(i, n) for(int i=0;i<n;i++)
#define all(i, v) for(auto& i : v)
typedef long long ll;
using namespace std;
const ll mod=1000000007, INF=(1LL<<60);
#define doublecout(a) cout<<fixed<<setprecision(10)<<a<<endl;

int main() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    int cnt = 0;
    string t = "AGCT";
    string s;
    cin >> s;
    rep(i, n) {
        rep(j, 4) if(s[i] == t[j]) cnt++;
    }
    if(cnt == 0) {
        cout << 0 << endl;
        return 0;
    }
    int id1 = cnt - 1, id2 = cnt - 1, p = cnt - 1;
    rep(i, 4) if(t[i] == s[p]) cnt--;
    while(cnt > 0) {
        bool f = false;
        rep(i, 4) if(t[i] == s[p]) {
            f = true;
        }
        if(f) {
            p = id1 - 1;
            id1--;
        } else {
            p = id2 + 1;
            id2++;
        }
        rep(i, 4) if(t[i] == s[p]) {
            cnt--;
        }
    }
    cout << id2 - id1 + 1 << endl;
    return 0;
}