#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int n, m;
  cin >> m >> n;
  int ans = 0;
  int a = m / 2;
  ans = n * a;
  if (m % 2 == 0)
    cout << ans << endl;
  else
    cout << ans + n / 2 << endl;
  return 0;
}
