#include <bits/stdc++.h>
int main() {
  int M, N, a = 2, b = 1, w, x, y, z;
  scanf("%d%d", &M, &N);
  z = 0;
  x = M / a;
  y = N / b;
  if (M % a == 0 && N % b == 0)
    w = x * y;
  else if (M % a != 0)
    z = ((M % a) * N) / 2;
  w = (x * y) + z;
  printf("%d", w);
  return 0;
}
