#include <bits/stdc++.h>
using namespace std;
int main() {
    long long a, b; cin >> a >> b;
    long long d = a + b;
    long long ans = 1e18;
    for (long long c = 1; c * c <= d; c++) {
        if (d % c == 0) {
            if ((a + c) % b == 0 && (b + c) % a == 0) {
                if (c != a && c != b) ans = min(ans, c);
            } 
        }
        long long e = d / c;
        if (d % e == 0) {
            if ((a + e) % b == 0 && (b + e) % a == 0) {
                if (e != a && e != b) ans = min(ans, e);
            } 
        }
    }
    if (ans == 1e18) cout << -1 << endl;
    else cout << ans << endl;
    return 0;
}