#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const int MAXN = 100 + 5;
int a[MAXN];
int main() {
  std::ios::sync_with_stdio(false);
  int a, b;
  cin >> a >> b;
  cout << a * b / 2 << endl;
  return 0;
}
