#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  if (n > m) {
    int t = n;
    n = m;
    m = t;
  }
  if (n == 1)
    printf("%d\n", m);
  else if (n == 2) {
    m <<= 1;
    printf("%d\n", ((m >> 3) << 2) + min(4, (m & 0x7)));
  } else {
    n *= m;
    n += (n & 1);
    printf("%d\n", n >> 1);
  }
  return 0;
}
