#include <bits/stdc++.h>
using namespace std;
string s;
long long b, c, n, d, l, i, k, ma, mi, sum, e, f, m, g, day;
long long a[100007];
int main() {
  cin >> n >> m;
  k = n * m;
  cout << k / 2;
  return 0;
}
