#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
    ll a,b;cin>>a>>b;
    ll u=a+b;
    vector<ll> v;
    for(ll i=1;i*i<=u;i++){
        if(u%i==0){
            v.push_back(i);
            v.push_back(u/i);
        }
        
    }
    sort(v.begin(),v.end());
    for(ll i=0;i<v.size();i++){
        if((v[i]+a)%b==0&&(v[i]+b)%a==0){
            cout<<v[i]<<endl;
            return 0;
        }
    }
    
    cout<<-1<<endl;
}