#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main() {
    int N;
    string s;
    cin >> N >> s;
    vector sum(3, vector<ll>(N+1, 0));
    for(int i = 0; i < N; i++) {
        int t = (s[i] == 'A' ? 0 : (s[i] == 'C' ? 1 : 2));
        for(int j = 0; j < 3; j++) sum[j][i + 1] = sum[j][i] + (j == t);
    }
    ll now = 0;
    for(int i = 0; i < N; i++) {
        if(s[i] == 'A') {
            now += (sum[1][N] - sum[1][i+1]) + (sum[2][N] - sum[2][i+1]);
        }
    }
    ll ans = now;
    for(int i = 0; i < N; i++) {
        if(s[i] == '?') {
            now -= sum[0][i] + sum[2][i];
            now += (sum[1][N] - sum[1][i+1]) + (sum[2][N] - sum[2][i+1]);
        }
        ans = max(ans, now);
    }
    cout << ans << endl;
}