#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int (i)=(0);(i)<(int)(n);++(i))
using ll = long long;
using P = pair<ll, ll>;
using namespace std;

template<class T> void vin(vector<T>& v, int n) {
    v.resize(n);
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
    }
}

// 約数の列挙
vector<ll> divisor(ll n) {
    vector<ll> res;
    for (ll i=1;i*i<=n;++i) {
        if (n%i == 0) {
            res.push_back(i);
            if (i != n/i) res.push_back(n/i);
        }
    }
    return res;
}

int main()
{
    ll A, B;
    cin >> A >> B;
    vector<ll> tmp = divisor(A+B);

    for (ll t : tmp) {
        if ((A+B)%t == 0 and (A+t)%B == 0 and (B+t)%A == 0) {
            cout << t << endl;
            return 0;
        }
    }

    cout << -1 << endl;
    return 0;
}