#include <bits/stdc++.h>
int main(void) {
  int r, c, domino;
  scanf("%d%d", &r, &c);
  if (r == 1 && c == 1) {
    domino = 0;
  } else if (r == 1 && c != 1) {
    domino = c / 2;
  } else if (c == 1 && r != 0) {
    domino = r / 2;
  } else if (r % 2 == 0) {
    domino = (r / 2) * c;
  } else {
    if (c % 2 == 0) {
      domino = (c / 2) * r;
    } else
      domino = ((c / 2) * r) + (r / 2);
  }
  printf("%d", domino);
  return 0;
}
