#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll,ll>;
#define fix(x) fixed << setprecision(x)
#define asc(x) x, vector<x>, greater<x>
#define rep(i, n) for(ll i = 0; i < n; i++)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}
constexpr ll INFLL = (1LL << 62), MOD = 998244353;
constexpr int INF = (1 << 30);

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n; string s;
    cin >> n >> s;
    ll ans = 0, upd = 0, cnt = 0, a = 0;
    rep(i,n){
        ans += (s[i]=='C') * cnt;
        cnt += (s[i]!='C');
    }
    a = cnt, upd = ans, cnt = 0;
    for(int i=n-1;i>=0;i--){
        cnt += (s[i]=='A');
        if(s[i]=='?'){
            upd -= cnt;
            cnt++;
            upd += a - cnt;
        }
        chmax(ans, upd);
    }
    cout << ans << '\n';
    return 0;
}