题解

#include <iostream>
#include <vector>
using namespace std;
int main() {
  int N;
  string S;
  cin >> N >> S;
  long long NC = 0;
  long long NO = 0;
  long long NW = 0;
  for (int i = 0; i < N; i++) {
    if (S[i] == 'C') {
      NC++;
    } else if (S[i] == 'O') {
      NO += NC;
    } else if (S[i] == 'W') {
      NW += NO;
    }
  }
  cout << NW << endl;
  return 0;
}