#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, v;
  long long ans = 0;
  char c;
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    cin >> c;
    v = c - 48;
    ans += (v % 2 == 0) * i;
  }
  cout << ans << "\n";
  return 0;
}
