#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int a, b;
    cin >> a >> b;
    int c = -1;
    for (int i = 1; i * i <= a + b; i++) {
        if (i == a || i == b) continue;
        if ((a + b) % i == 0 && (i + b) % a == 0 && (i + a) % b == 0) {
            c = i;
            break;
        }
    }
    cout << c << endl;
    return 0;
}