#include <bits/stdc++.h>
using namespace std;
const double EPS = (double)1e-9;
const double pii = acos(-1);
const int mod = 1000000007;
const int N = (int)1e5;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  int m, n;
  cin >> m >> n;
  int ans = (n * m) / 2;
  cout << ans << "\n";
}
