#include <bits/stdc++.h>
#define rep(i, n) for(long long i = 0; i < (long long)(n); i++)
using namespace std;
using ll = long long;
const ll INF = 1e18;

// 約数列挙
vector<long long> calc_divisor(long long n) {
    vector<long long> res;
    for (long long i = 1LL; i*i <= n; ++i) {
        if (n % i == 0) {
            res.push_back(i);
            long long j = n / i;
            if (j != i) res.push_back(j);
        }
    }
    sort(res.begin(), res.end());
    //sort(res.rbegin(), res.rend());
    return res;
}


int main(){
    ll a, b;
    cin >> a >> b;

    auto d = calc_divisor(a+b);

    ll ans = INF;
    rep(i, d.size()){
        ll c = (a + b) / d[i];
        if(a != c && b != c && (a + c) % b == 0 && (b + c) % a == 0) ans = min(ans, c);
    }

    if(ans == INF) cout << "-1" << endl;
    else cout << ans << endl;

    return 0;
}