#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 1000000000000000001



int main(){
	
	int n;
	cin>>n;
	string S;
	cin>>S;
	vector<int> pos;
	long long cs = 0,cc = 0;
	rep(i,n){
		if(S[i]=='C'){
			cs -= cc;
			cs += i;
			cc++;
			
		}
		if(S[i]=='?'){
			pos.push_back(i);
		}
	}
	long long ans = cs;
	reverse(pos.begin(),pos.end());
	rep(i,pos.size()){
		cs += pos[i];
		cs -= cc;
		cc++;
		ans = max(ans,cs);
	}
	cout<<ans<<endl;
	return 0;
}