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