Problem:
Score : 100 points

Problem Statement

You are given positive integers X and Y.
If there exists a positive integer not greater than 10^{18} that is a multiple of X but not a multiple of Y, choose one such integer and print it.
If it does not exist, print -1.

Constraints

1 ≤ X,Y ≤ 10^9

X and Y are integers.

Input

Input is given from Standard Input in the following format:

X Y

Output

Print a positive integer not greater than 10^{18} that is a multiple of X but not a multiple of Y, or print -1 if it does not exist.

Sample Input 1

8 6

Sample Output 1

16

For example, 16 is a multiple of 8 but not a multiple of 6.

Sample Input 2

3 3

Sample Output 2

-1

A multiple of 3 is a multiple of 3.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;

class MainClass
{
    
    public static void Main(string[] args)
    {


        //long N = long.Parse(Console.ReadLine());
        //long[] a = new long[N];
        //long[] b = new long[N];

        //for (int i = 0; i < N; i++)
        //{
        //    string[] s1 = Console.ReadLine().Split(' ');
        //    a[i] = long.Parse(s1[0]);
        //    b[i] = long.Parse(s1[0]);


        //}

        string[] s1 = Console.ReadLine().Split(' ');
        long x = long.Parse(s1[0]);
        long y = long.Parse(s1[1]);


        if(x == y)
        {
            Console.WriteLine("-1");
        }
        else if(x%y == 0)
        {
            Console.WriteLine("-1");
        }
        else
        {
            Console.WriteLine(x);
        }

        //Console.WriteLine("Yes");
        //Console.WriteLine("No");

    }

}