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