#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int a, b;
  cin >> a >> b;
  long long int c = a * b;
  if (c % 2 == 0) {
    cout << c / 2 << "\n";
    return 0;
  }
  cout << c / 2 + 1 << "\n";
}
