#include <iostream>

using namespace std;

int main()
{
    ifstream in("cow.in");
    ofstream out("cow.out");
    int n;
    cin>>n;
    char input;
    long long c=0;
    long long co=0;
    long long result=0;

    cin>>input;

    for(int i=0;i<n;i++)
    {
        cin>>input;
        if(input=='C')
            c++;
        else if(input=='O')
            co+=c;
        else
            result+=co;
    }
    cout<<result;
    return 0;
}