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: Runtime Error)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using static System.Console;
using static System.Math;
namespace CP
{
    class Atria
    {
        static void Main(string[] args)
        {
            int[] s = ReadLine().Split(' ').Select(int.Parse).ToArray();
            int x = s[0];
            int y = s[1];
            var yy = PrimeFactor(y).ToArray();
            if (yy.Count() == 1)
            {
                foreach (var item in yy)
                {
                    if (x % item == 0)
                    {
                        WriteLine(-1);
                        return;
                    }
                }
                WriteLine(x);
                return;
            }
            WriteLine(
                x * yy[0]
                );
        }

        static IEnumerable<long> PrimeFactor(long n)
        {
            int i = 2;
            long tmp = n;

            while (i <= Math.Sqrt(n))
            {
                if (tmp % i == 0)
                {
                    tmp /= i;
                    yield return i;
                }
                else i++;
            }
            if (tmp != 1) yield return tmp;
        }
    }
}