#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
using ll = long long;

int main(void){
  ll a, b; cin >> a >> b;
  if(a>b) swap(a, b);
  if(b%3==0){
    ll x=b/3;
    if(x*2==a){
      cout << x << endl; return 0;
    }
    if(b/3==a){
      cout << a*2 << endl; return 0;
    }
  }
  if(a*2==b){
    cout << a*3 << endl; return 0;
  }
  cout << -1 << endl;
  return 0; 
}