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