#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int x, y;
  cin >> x >> y;
  long long int count1 = x * y;
  count1 = count1 / 2;
  cout << count1 << endl;
  return 0;
}
