#include <bits/stdc++.h>
using namespace std;
int main() {
  int m, n;
  cin >> n >> m;
  int r;
  int s = n * m;
  if (s % 2 == 0) {
    r = s / 2;
  }
  if (s % 2 == !0) {
    r = (s - 1) / 2;
  }
  cout << r;
  return 0;
}
