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

vector<long long> enum_divisor(long long n){
    vector<long long> ret;
    for (long long i = 1; i * i <= n; ++i){
        if (n % i == 0){
            ret.push_back(i);
            if (i * i != n){
                ret.push_back(n / i);
            }
        }
    }
    return ret;
}

int main()
{
    ios::sync_with_stdio(false);
    cout.tie(0);
    long long A, B;
    cin >> A >> B;
    // a + b = k1 * c
    // b + c = k2 * a  -> (k1*c-a)+c=k2*a ->(k1+1)*c = (k2+1)*a
    // c + a = k3 * b  -> (c + k1*c-b) = k3*b => (k1+1)*c = (k3+1)*b
    // (k2+1)*a = (k3+1)*b
    auto cs = enum_divisor(A + B);
    long long ans = A + B + 1;
    for (auto c: cs){
        if ((B + c) % A == 0 and (c + A) % B == 0 and c < ans and c != A and c != B) ans = c;
    }
    if (ans == A + B + 1) ans = -1;
    cout << ans << endl;
    return 0;
}