#include <bits/stdc++.h>
int n, m, ans;
int main() {
  scanf("%d%d", &n, &m);
  if (n > m) n ^= m ^= n ^= m;
  switch (n) {
    case 1: {
      ans = (m / 3 + (m % 3 ? 1 : 0));
      break;
    }
    case 2: {
      ans = ((m >> 1) + 1);
      break;
    }
    case 3: {
      ans = (m - ((m - 1) >> 2));
      break;
    }
    case 4: {
      if (m == 4 || m == 7 || m == 8 || m == 10)
        ans = m;
      else
        ans = m + 1;
      break;
    }
    case 5: {
      if (m >= 5 && m <= 7)
        ans = (m + 2);
      else if (m == 8)
        ans = 11;
      break;
    }
    default: {
      ans = 10;
      break;
    }
  }
  printf("%d\n", n * m - ans);
  return 0;
}
