#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;
using ll = long long;

int main(){

    ll N, ans=0, cntc=0, now=0, cnta=0;
    string S;
    cin >> N >> S;

    for (int i=0; i<N; i++){
        if (S[i] == 'C') ans += cnta;
        else cnta++;
    }
    now = ans;
    
    for (int i=N-1; i>=0; i--){
        if (S[i] == 'C') cntc++;
        else if (S[i] == '?'){
            now -= cntc;
            cntc++;
            cnta--;
            now += cnta;
        }
        else cnta--;
        ans = max(ans, now);
    }

    cout << ans << endl;

    return 0;
}