#include<iostream>
#include<vector>
#include<algorithm>
#include<list>

#define REP(i,a,n) for(int i=a;i<n;i++)

typedef long long ll;
using namespace std;


vector<long long> prime_factor(ll n){
    vector<long long> Clist;
    for(int i=1;i*i <= n;i++){
        if(n%i==0){
            Clist.push_back(i);
            if((n/i)==i)continue;
            Clist.push_back(n/i);
        }
    }
    sort(Clist.begin(),Clist.end());
    return Clist;
}

int main(){
    long long A,B,ans=-1;
    cin >> A >> B;
    vector<long long> pf;
    pf=prime_factor(A+B);
    
    /**
    for(auto itr=Clist.begin();itr!=Clist.end();itr++){
        cout << *itr << " ";
    }
    cout << endl;
    **/
    
    for(auto itr=pf.begin();itr!=pf.end();itr++){
        if((A+*itr)%B!=0)continue;
        if((B+*itr)%A!=0)continue;
        if(A==*itr||B==*itr)continue;
        ans=*itr;
        break;
    }
    cout << ans << endl;
   
}