#include <bits/stdc++.h>

using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    string s;
    cin >> n >> s;
    vector<long long> cntA(n + 1, 0), cntC(n + 1, 0);
    vector<long long> cntQ(n + 1, 0), cntQ2(n + 1, 0);
    for (int i = 0; i < n; i++) {
        cntA[i + 1] = cntA[i] + (s[i] == 'A');
    }
    for (int i = n - 1; i >= 0; i--) {
        cntC[i] = cntC[i + 1] + (s[i] == 'C');
    }
    for (int i = n - 1; i >= 0; i--) {
        cntQ[i] = cntQ[i + 1] + (s[i] == '?');
    }
    for (int i = 0; i < n; i++) {
        cntQ2[i + 1] = cntQ2[i] + (s[i] == '?');
    }
    long long ans = 0;
    long long cur = 0;
    for (int i = 0; i < n; i++) {
        if (s[i] == 'A') {
            cur += cntC[i] + cntQ[i];
        }
    }
    ans = cur;
    for (int i = 0; i < n; i++) {
        if (s[i] == '?') {
            cur -= cntA[i] + cntQ2[i];
            cur += cntQ[i + 1] + cntC[i + 1];
            ans = max(ans, cur);
        }
    }
    cout << ans << '\n';
}