#include <algorithm>
#include <cmath>
#include <complex>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <vector>

using namespace std;

using ll = long long;
using P = pair<int, int>;

const long long MOD = 1e9+7;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

vector<int> divisor(int n) {
    vector<int> res;
    for (int 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() {
    int a, b;
    cin >> a >> b;
    vector<int> ds = divisor(a + b);
    for (auto d : ds) {
        if (d == a || d == b) continue;
        if ((a + d) % b == 0 && (b + d) % a == 0) {
            cout << d << endl;
            return 0;
        }
    }
    cout << -1 << endl;
}