#include <bits/stdc++.h>
using namespace std;
int main() {
  int m;
  int n;
  cin >> m >> n;
  int res, res1, res2;
  res1 = (m / 2) * n + (m % 2) * (n / 2);
  res2 = (n / 2) * m + (n % 2) * (m / 2);
  res = max(res1, res2);
  cout << res;
}
