#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, sum1, sum2;
  cin >> n >> m;
  sum1 = n / 2 * m;
  sum2 = m / 2 * n;
  if (n % 2 == 1) sum1 += m / 2;
  if (m % 2 == 1) sum2 += n / 2;
  cout << max(sum1, sum2);
  return 0;
}
