#include <bits/stdc++.h>

using namespace std;

bool isValid(int64_t A, int64_t B, int64_t C) {
    //  A + B
    int64_t plus = A + B;
    int64_t other = C;
    if (plus < other) swap(plus, other);
    if (plus % other != 0) return false;

    // B + C
    plus = B + C;
    other = A;
    if (plus < other) swap(plus, other);
    if (plus % other != 0) return false;

    // C + A
    plus = C + A;
    other = B;
    if (plus < other) swap(plus, other);
    if (plus % other != 0) return false;

    return true;
}

int main(int argc, char *argv[]) {
    int64_t A, B;
    cin >> A >> B;

    int64_t plus = A + B;

    int64_t ans = INT64_MAX;
    for (int64_t i = 1; i * i <= plus; i++) {
        if (plus % i == 0) {
            if (A != i && B != i && isValid(A, B, i)) {
                ans = min(ans, i);
            }
            int64_t other = plus / i;
            if (A != other && B != other && isValid(A, B, other)) {
                ans = min(ans, other);
            }
        }
    }
    cout << ((ans == INT64_MAX) ? -1 : ans) << endl;
	return 0;
}