#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long x, y, ans;
  cin >> x >> y;
  ans = (x / 2) * y;
  if (x % 2) ans += y / 2;
  cout << ans;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  long long t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
