#include <bits/stdc++.h>
using namespace std;
int main(void) {
  long long a, b, ans;
  cin >> a >> b;
  if (a % 2 == 0)
    ans = a * b / 2;
  else if (b % 2 == 0)
    ans = a / 2 * b;
  else
    ans = a / 2 * b + b / 2 + 1;
  printf("%lld", ans);
  return 0;
}
