#include<bits/stdc++.h>
using namespace std;
int main()
{
    int A, B;
    int C = -1;
    cin >> A >> B;
    for(int n = 1; n <= A + B; n++)
    {
        if ((A + n)%B==0 && (B + n)%A==0 && A!= B && A != n && B != n)
        {
            C = n;
        }
    }
    cout << C << endl;
    return 0;
}