#include <bits/stdc++.h>
using namespace std;
int z[1000][1000], g = 0;
int main() {
  int a, b;
  cin >> a >> b;
  if ((a * b) % 2 == 1) {
    cout << ((a * b) / 2);
  } else
    cout << (a * b) / 2;
}
