#include <bits/stdc++.h>
using namespace std;
long long T, n, m, x, y;
int main() {
  scanf("%I64d %I64d", &x, &y);
  n = min(x, y);
  m = max(x, y);
  if (n == 2) {
    long long tmp = m / 4;
    long long res = m % 4;
    tmp = tmp * 4;
    if (res == 1) tmp += 2;
    if (res == 2) tmp += 4;
    if (res == 3) tmp += 4;
    printf("%I64d\n", tmp);
  } else if (n == 1)
    printf("%I64d\n", n * m);
  else
    printf("%I64d\n", (n * m + 1) / 2);
  return 0;
}
