#include <bits/stdc++.h>
using namespace std;
int main() {
  unsigned long long h, w, ans;
  cin >> h >> w;
  ans = h * w;
  if (ans % 2 == 1) {
    ans--;
    ans /= 2;
    ans++;
  } else
    ans /= 2;
  cout << ans;
}
