#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, sum, mul = 0;
  cin >> a;
  cin >> b;
  mul = a * b;
  sum = mul / 2;
  if (mul % 2 == 0)
    cout << sum;
  else if (mul % 2 == 1)
    cout << sum;
  return 0;
}
