#ifdef LOCAL
#include <local.hpp>
#else
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2")
#include <bits/stdc++.h>
#define debug(...) ((void)0)
#define postprocess(...) ((void)0)
#endif

#include "atcoder/segtree.hpp"

using namespace std;
using ll = long long;
using ld = long double;

using Sclass = pair<ll, ll>;

Sclass op(Sclass a, Sclass b) {
    return {a.first + b.first, a.second + b.second};
}
Sclass e() {
    return {0, 0};
}

void solve([[maybe_unused]] ll test) {
    ll N;
    cin >> N;

    string S;
    cin >> S;

    vector<pair<ll, ll>> a(N);
    for (ll i = 0; i < N; i++) {
        switch (S[i]) {
            case 'A':
                a[i] = {1, 0};
                break;

            case 'C':
                a[i] = {0, 1};
                break;

            default:
                a[i] = {0, 1};
                break;
        }
    }

    atcoder::segtree<Sclass, op, e> st(a);

    ll ans = 0;

    ll tmp = 0;
    for (ll i = 0; i < N; i++) {
        if (S[i] == 'A') tmp += st.prod(i + 1, N).second;
    }
    ans = max(ans, tmp);

    for (ll i = 0; i < N; i++) {
        if (S[i] != '?') continue;

        st.set(i, {1, 0});

        tmp -= st.prod(0, i).first;
        tmp += st.prod(i + 1, N).second;

        ans = max(ans, tmp);
    }

    cout << ans << endl;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    // cin >> t;
    for (int i = 1; i <= t; i++) {
        solve(i);
    }

    postprocess();
}