#include <bits/stdc++.h>
int main() {
  int n, m, x;
  scanf("%d%d", &n, &m);
  if (n > m) x = n, n = m, m = x;
  if (n == 1)
    x = m / 3 + (m % 3 != 0);
  else if (n == 2)
    x = m / 2 + 1;
  else if (n == 3)
    x = m - (m - 1) / 4;
  else if (n == 4)
    x = (m + 1) - (m == 4 or m == 7 or m == 8 or m == 10);
  else if (n == 5)
    x = 7 * (m == 5) + 8 * (m == 6) + 9 * (m == 7) + 11 * (m == 8);
  else if (n == 6)
    x = 10;
  printf("%d\n", n * m - x);
  return 0;
}
