#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

int main(){
    int A,B;
    cin>>A>>B;
    int ans=-1;
    vector<int> divisors(0);
    vector<int> divisors_rev(0);
    for(int C=1;C*C<=A+B;C++){
        if((A+B)%C==0){
            divisors.push_back(C);
            divisors_rev.push_back((A+B)/C);
        }
    }
    reverse(divisors_rev.begin(),divisors_rev.end());
    for(int d:divisors_rev){
        divisors.push_back(d);
    }
    for(int d:divisors){
        if(A!=d&&B!=d&&(A+d)%B==0&&(B+d)%A==0){
            ans=d;
            break;
        }
    }
    cout<<ans<<endl;
}