#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int r, c, res;
  cin >> r >> c;
  if (r % 2 == 0) {
    res = r * (c / 2);
  } else {
    res = r * (c / 2) + (r + 1) / 2;
  }
  cout << res;
}
