#include <iostream>
using namespace std;
typedef long long ll;
int main(){
  ll r,c,x;
  cin>>r>>c;
  x=r*c;
  x-=1;
  if(x>1) x/=2;
  if(x>1 && r==c) x/=2;
  cout<<x<<endl;
  return 0;
}