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

#ifdef _RUTHEN
#include <debug.hpp>
#else
#define show(...) true
#endif

#define REP(i, n) for (int i = 0; i < (n); i++)
template <class T> using V = vector<T>;

#include <atcoder/modint>
using mint = atcoder::modint998244353;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int N;
    string S;
    cin >> N >> S;
    long long ans = 0, ca = 0;
    REP(i, N) {
        if (S[i] == 'A') {
            ca++;
        } else if (S[i] == 'C') {
            ans += ca;
        } else {
            ca++;
        }
    }
    show(ans, ca);
    long long s = ans, rc = 0;
    for (int i = N - 1; i >= 0; i--) {
        if (S[i] == '?') {
            // S[i] = 'A' => S[i] = 'C'
            ca--;
            s -= rc;
            s += ca;
            rc++;
            ans = max(ans, s);
        } else if (S[i] == 'A') {
            ca--;
        } else {
            rc++;
        }
        show(ans, s, ca, rc);
    }
    cout << ans << '\n';
    return 0;
}