#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
typedef long long ll;
int main(){
    ll a, b;
    cin>>a>>b;
    vector<ll> d;
    for(ll c=1; c*c<=a+b; c++){
        if((a+b)%c==0){
            d.push_back(c);
            if(c*c<a+b) d.push_back((a+b)/c);
        }
    }
    sort(d.begin(), d.end());
    for(auto c:d){
        if((a+c)%b==0 && (b+c)%a==0 && c!=a && c!=b){
            cout<<c<<endl;
            return 0;
        }
    }
    cout<<-1<<endl;
    return 0;
}