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