#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define pb push_back
#define mp make_pair
#define rep(i,n) for(int i=0;i<(n);++i)
const int mod=1000000007;

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	ll a,b;cin >> a >> b;
	if(a>b){
		ll c=a;
		a=b;
		b=c;
	}
	if(a*2==b){
		cout << a*3 << endl;
	}
	else if(a*3==b){
		cout << a*2 << endl;
	}
	else if((a+b)%(b-a)==0 && b%(b-a)==0 && a%(b-a)==0){
		cout << b-a << endl;
	}
	else{
		cout << -1 << endl;
	}
}