#include<bits/stdc++.h>
using namespace std;

void solve()
{
    int a, b;
    cin >> a >> b;

    for (int x = 1; x <= 100000; ++x)
    {
        if (x % a == b % x)
        {
            cout << x << "\n";
            return;
        }
    }
    cout << -1 << "\n";
}


int main(int argc, char *argv[])
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    solve();
    return 0;
}