#50A - Domino piling.cpp

//link to problem https://codeforces.com/problemset/problem/50/A
#include <iostream>
 
using namespace std;
 
int main () {
  int m, n, count = 0;
  cin >> m >> n;
  count = (m * n) / 2;
  cout << count;
 
  return 0;
}