#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  scanf("%d %d", &n, &m);
  if (n > m) swap(n, m);
  int ans;
  if (n == 1) {
    ans = m;
  } else if (n == 2) {
    ans = 2 * ((m / 4) * 2 + min(m % 4, 2));
  } else {
    ans = max(ans, n * m - n * m / 2);
  }
  printf("%d\n", ans);
  return 0;
}
