#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <set>
#include <random>
#include <iomanip>
#include <string>
#include <cmath>
#include <complex>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)

template<class T>
using vi = vector<T>;

template<class T>
using vii = vector<vi<T>>;

template<class T>
using viii = vector<vii<T>>;

using P = pair<ll, int>;

void chmin(ll & x, ll y) { x = min(x, y); }

void chmax(ll& x, ll y) { x = max(x, y); }

ll calc(int n, string& s) {
    ll res = 0;
    ll ca = 0;
    rep(i, n) {
        if (s[i] == 'C') res += ca;
        else ca++;
    }
    return res;
}

string mk(int n, string s, int m) {
    string res = s;
    int cnt = 0;
    rep(i, n) {
        if (s[i] == '?') {
            if (cnt <= m) res[i] = 'A';
            else res[i] = 'C';
            cnt++;
        }
    }
    return res;
}

int main()
{
    int n; string s;
    cin >> n >> s;
    int mx = 0;
    rep(i, n) if (s[i] == '?') mx++;

    int ok = 0, ng = mx + 1;
    while (ng - ok > 2) {
        int m1 = ok + (ng - ok) / 3;
        int m2 = ok + (ng - ok) / 3 * 2;
        string s1 = mk(n, s, m1);
        string s2 = mk(n, s, m2);
        if (calc(n, s1) < calc(n, s2)) ok = m1;
        else ng = m2;
    }

    ll ans = 0;
    for (int i = ok; i < ng; i++) {
        string ns = mk(n, s, i);
        chmax(ans, calc(n, ns));
    }
    cout << ans << endl;
    return 0;
}