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

int main(){
    long A, B;
    cin >> A >> B;
    long N{A + B};
    vector<long> tapi, tape;
    {
        long i;
        for(i = 1; i * i <= N; ++i)if(N % i == 0){tapi.push_back(i); tape.push_back(N / i);}
    }
    reverse(tape.begin(), tape.end());
    //for(const auto i : tapi)
    //    if((A + i) % B == 0 && (B + i) % A == 0)return 0 & printf("%ld\n", i);
    //for(const auto i : tape)
    //    if((A + i) % B == 0 && (B + i) % A == 0)return 0 & printf("%ld\n", i);
    puts("-1");
}