#include <bits/stdc++.h>
int main() {
  int m, n, hsl, tamp;
  scanf("%d %d", &m, &n);
  if (m >= 1 && m <= 16 && n >= 1 && n <= 16 && m <= n) {
    tamp = (m * n) / 2;
    printf("%d", tamp);
  } else
    ;
  return 0;
}
