#include<bits/stdc++.h>
using namespace std;
#define ll long long
typedef pair<ll,ll> P;
#define M 1000000007
int main(){
  ll a,b;
  cin>>a>>b;
  if(max(a,b)==min(a,b)*3) cout<<min(a,b)*2<<endl;
  else if(max(a,b)==min(a,b)*2) cout<<min(a,b)*3<<endl;
  else if(max(a,b)*2==min(a,b)*3) cout<<min(a,b)/2<<endl;
  else cout<<-1<<endl;
}