#include <bits/stdc++.h>
using namespace std;
void solve() {
  int w, h, ans = 0;
  cin >> w >> h;
  if (w % 2 != 0) ans += h % 2 == 0 ? h / 2 : (h - 1) / 2;
  while (w != 0 && w > 1) {
    for (int j = 1; j <= h; j++) {
      ans++;
    }
    w -= 2;
    if (w == 1) break;
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int tc = 1;
  while (tc--) {
    solve();
  }
}
