#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
signed main(){
  Int h,w;
  cin>>h>>w;
  Int ans=0;
  if(h==w){
    ans=(h/2)*(w/2);
    if(h&1) ans+=(w+1)/2;
  }else{
    ans=h*w/2; 
    if((h&1)&&(w&1)) ans++;
  }
  cout<<ans-1<<endl;
  return 0;
}