#include<bits/stdc++.h>
using namespace std;
using LL=long long;
using ULL=unsigned long long;
#define rep(i,n) for(int i=0;i<(n);i++)

int main(){
  LL N,M; cin>>N>>M;
  LL ans=0;
  while(true){
    if(N%2==0 && M%2==0){
      ans=0; break;
    }
    if(N%2==1) swap(N,M);
    if(N%2==1){
      if(N%4!=3 || M%4!=3){ ans=N/4+M/4; break; }
      if(N==3 || M==3){ ans=N/4+M/4; break; }
      ans=N/4+M/4+2; break;
    }
    if(N%4==0){ ans=N/4; break;}
    ans=0; break;
  }
  
  cout<<ans<<endl;

  return 0;
}