#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b;
  while (cin >> a >> b) {
    int m, n;
    int total = 0;
    if (a > b) {
      m = a;
      n = b;
    } else {
      m = b;
      n = a;
    }
    total = m / 2;
    total = total * n + (m - total * 2) * n / 2;
    cout << total << endl;
  }
  return 0;
}
