#include <bits/stdc++.h>
using namespace std;
int n, m, aux, aux1;
int variabila, variabila1, nr;
int main() {
  cin >> n >> m;
  aux = (n / 2) * m;
  aux1 = (m / 2) * n;
  if (n % 2) {
    variabila = aux + m / 2;
    nr++;
  }
  if (m % 2) {
    variabila1 = aux1 + n / 2;
    nr++;
  }
  if (nr)
    cout << max(variabila, variabila1);
  else
    cout << aux;
  return 0;
}
