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