#include <bits/stdc++.h>
using namespace std;
#define int long long
#define fi first
#define se second
#define rep(i, s, n) for (int i = s; i < n; i++)
#define rrep(i, s, n) for (int i = (n)-1; i >= (s); i--)
const long long MOD = 1e9 + 7, INF = 1e18;

signed main()
{
    int A, B;
    cin >> A >> B;

    int t = A + B;
    for (int i = 1; i * i <= t; i++)
    {
        if ((A + B) % i == 0)
        {
            if (((A + i) % B == 0 && (B + i) % A == 0) && (A != i && B != i))
            {
                cout << i << endl;
                return 0;
            }
        }
    }
    for (int i = 1; i * i <= t; i++)
    {
        if ((A + B) % i == 0)
        {
            int j = (A + B) / i;
            if (((A + j) % B == 0 && (B + j) % A == 0) && (A != j && B != j))
            {
                cout << j << endl;
                return 0;
            }
        }
    }
    cout << -1 << endl;
}