#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, temp, ans;
  cin >> n >> m;
  temp = n * m;
  if (temp % 2 == 0) {
    ans = temp / 2;
  } else {
    ans = (temp - 1) / 2;
  }
  cout << ans << endl;
}
