#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define fi first
#define se second

vector<ll> finddivisor(ll x){
    vector<ll> divisor;
    for(ll i=1; (i*i)<=x ;i++)
    {
        if(x%i==0){
            divisor.push_back(i);
            if(i*i!=x){
                divisor.push_back(x/i);
            }//約数はペアを持つことを利用

        }
    }
    sort(divisor.begin(),divisor.end());
    divisor.push_back(x);
    return divisor;
}//整数xの約数が入ったvectorを返すよ(xを含む)  

int main(){
    ll a,b;
    cin >> a >> b;
    vector<ll> v=finddivisor(a+b);
    

    for(ll i=0;i < v.size();i++)
    {
        if((b+v.at(i))%a == (v.at(i)+a)%b && (b+v.at(i))%a==0 && v.at(i)!=a && b!=v.at(i)){
            cout << v.at(i) << endl;
            return 0;
        }
    }
        
    cout << -1 << endl;
}