#include <bits/stdc++.h>
using namespace std;
int main() {
  int first, second;
  scanf("%d%d", &first, &second);
  if (first > second) swap(first, second);
  int sum = 0;
  if (first <= 2) {
    if (first == 1)
      sum = second;
    else {
      if (second < 5)
        sum = 4;
      else {
        if (second % 2 == 0) {
          if ((second - 6) % 4 == 0)
            sum = second + 2;
          else
            sum = second;
        } else
          sum = second + 1;
      }
    }
  } else {
    if (first % 2 == 0 || second % 2 == 0)
      sum = (first * second) / 2;
    else
      sum = (first * second) / 2 + 1;
  }
  printf("%d\n", sum);
  return 0;
}
