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