#include <bits/stdc++.h>
int main() {
  int n, m, t;
  int a, b, c, d, count = 0;
  while (~scanf("%d%d", &n, &m)) {
    if (n == 1)
      printf("%d\n", m);
    else if (m == 1)
      printf("%d\n", n);
    else if (m == 2) {
      t = n / 4;
      if (n % 4 == 1)
        printf("%d\n", t * 4 + 2);
      else if (n % 4 == 0)
        printf("%d\n", t * 4);
      else
        printf("%d\n", t * 4 + 4);
    } else if (n == 2) {
      t = m / 4;
      if (m % 4 == 1)
        printf("%d\n", t * 4 + 2);
      else if (m % 4 == 0)
        printf("%d\n", t * 4);
      else
        printf("%d\n", t * 4 + 4);
    } else {
      if (n & 1) {
        a = n / 2;
        b = n - a;
        if (m & 1) {
          c = m / 2;
          d = m - c;
          count = b * d + a * c;
        } else
          count = n * (m / 2);
      } else {
        a = n / 2;
        count = a * m;
      }
      printf("%d\n", count);
    }
  }
  return 0;
}
