#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned long long ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}
inline double time() {
    return static_cast<long double>(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count()) * 1e-9;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n; cin >> n;
    string s; cin >> s;
    ll pa = 0, pc = 0;
    ll ba = 0, bc = 0;
    ll hatena = 0;
    ll res = 0;
    ll base = 0;
    for (int i = 0; i < n; ++i) {
        if (s[i] == 'A') {
            ba++;
        }
        else if (s[i] == 'C') {
            base += ba;
            bc++;
        }
        else {
            res += ba;
            hatena++;
        }
    }
    ll sum = res;
    ll ha = 0;
    for (int i = 0; i < n; ++i) {
        if (s[i] == 'A') {
            pa++; ba--;
        }
        else if (s[i] == 'C') {
            pc++; bc--;
        }
        else {
            ha++; hatena--;
            ll u = ha*hatena;
            sum -= pa;
            sum += bc;
            res = max(res, u+sum);
        }
    }
    cout << res+base << endl;
}