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