#include <bits/stdc++.h>
using namespace std;
int main() {
  int m, n, num = 0;
  cin >> m >> n;
  num += (m / 2) * n;
  if (m % 2 == 1) {
    num += (n / 2);
  }
  cout << num << "\n";
  return 0;
}
