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