#include <bits/stdc++.h>
using namespace std;
int main() {
  int m, n;
  cin >> m >> n;
  cout << ((m % 2) ? m * (n / 2) + (n % 2) * (m / 2)
                   : n * (m / 2) + (m % 2) * (n / 2));
  return 0;
}
