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

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