#include<iostream>
using namespace std;

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