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

int gcd(int a, int b)
{
    if (a == 1 || b == 1)
    {
        return min(a, b);
    }
    else 
    {
        return gcd(min(a, b), max(a, b) % min(a, b));
    }
}



int main()
{
    int A, B;
    int n;
    cin >> A >> B;
    n = gcd(A, B);
    A = A / n;
    B = B / n;
    if (min(A, B) == max(A, B) - 1)
    {
        cout << (min(A, B) + 2) * n <<endl;
    }
    else if (min(A, B) == max(A, B) - 2)
    {
        cout << (min(A, B) + 1) * n << endl;
    }
    else
    {
        cout << -1 << endl;
    }
    return 0;
}