#include <bits/stdc++.h>
int main(void) {
  int m, n;
  scanf("%d %d", &m, &n);
  if (n % 2 == 0) {
    printf("%d\n", m * (n / 2));
    return 0;
  } else {
    printf("%d\n", m * (n / 2) + (m / 2));
    return 0;
  }
  return 0;
}
