#include <bits/stdc++.h>
int main() {
  int a, b, greater, smaller;
  int count = 0;
  int cont = 0;
  scanf("%d %d", &a, &b);
  if (a > b) {
    smaller = b;
    greater = a;
  } else {
    smaller = a;
    greater = b;
  }
  int x = greater;
  for (int i = 0; i < smaller; i++) {
    while (greater - 2 >= 0) {
      greater = greater - 2;
      count++;
    }
    greater = x;
  }
  if (greater % 2 != 0) {
    while (smaller - 2 >= 0) {
      smaller = smaller - 2;
      cont++;
    }
  }
  printf("%d", count + cont);
  return 0;
}
