#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  string s;
  cin >> s;
  long long ret = 0;
  for (int i = 0; i < n; i++)
    if (s[i] == 'B') ret += (1LL << i);
  cout << ret << endl;
  return 0;
}
