#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, m;
  scanf("%I64d%I64d", &n, &m);
  if (n > m) swap(n, m);
  if (n == 1)
    printf("%I64d\n", m);
  else if (n == 2)
    printf("%I64d\n", m / 4 * 4 + (m % 4 == 3 ? 4 : m % 4 * 2));
  else {
    long long temp = m >> 1;
    long long haha = (n >> 1) * m + (n & 1) * (m - temp);
    printf("%I64d\n", max(n * m - haha, haha));
  }
  return 0;
}
