#include <bits/stdc++.h>

#define fst(t) std::get<0>(t)
#define snd(t) std::get<1>(t)
#define thd(t) std::get<2>(t)
#define unless(p) if(!(p))
#define until(p) while(!(p))

using ll = std::int64_t;
using P = std::tuple<int,int>;

int A, B;

bool f(int C){
    return (B + C) % A == 0 && (C + A) % B == 0;
}

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    std::cin >> A >> B;

    int sum = A + B;
    int C = -1;

    for(int i=1;i*i<=sum;++i){
        if(sum % i > 0){
            continue;
        }

        int j = sum / i;

        if(f(i) && i != A && i != B){
            if(C == -1 || i < C){
                C = i;
            }
        }

        if(f(j) && j != A && j != B){
            if(C == -1 || j < C){
                C = j;
            }
        }
    }

    std::cout << C << std::endl;
}