#include<bits/stdc++.h>
using namespace std;
int main()
{
    int A, B;
    int i;
    cin >> A >> B;
    for (i=max(A, B); i > 0; i--)
    {
        if (A % i == 0 && B % i == 0)
        {
            A /= i;
            B /= i;
            break;
        }
    }
    //cout << A << " " << B << " " << i << endl;
    
    if (min(A, B) == max(A, B) - 1)
    {
        cout << (min(A, B) + 2) * i << endl;
    }
    else if (min(A, B) == max(A, B) - 2)
    {
        cout << (min(A, B) + 1) * i << endl;
    }
    else 
    {
        cout << -1 << endl;
    }
    return 0;
}