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;
using System.Collections.Generic;
using System.Linq;

namespace APC001A_TwoIntegers
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] input = Console.ReadLine().Split(' ');

            int x = int.Parse(input[0]);
            int y = int.Parse(input[1]);

            int result = -1;

            if (x % y != 0)
            {
                for (int indexX = 1; indexX * x <= Math.Pow(10, 18); indexX++)
                {
                    int xx = indexX * x;
                    List<int> multipleY = new List<int>();
                    for (int indexY = 1; indexY * y <= xx; indexY++)
                    {
                        multipleY.Add(indexY * y);
                    }

                    if (!multipleY.Contains(xx))
                    {
                        result = xx;
                        break;
                    }
                }
            }

            //if (x != y)
            //{
            //    for (int indexX = 1; indexX * x <= Math.Pow(10, 18); indexX++)
            //    {
            //        int xx = indexX * x;
            //        List<int> multipleY = new List<int>();
            //        for (int indexY = 1; indexY * y <= xx; indexY++)
            //        {
            //            multipleY.Add(indexY * y);
            //        }

            //        if (!multipleY.Contains(xx))
            //        {
            //            result = xx;
            //            break;
            //        }
            //    }
            //}

            Console.WriteLine(result);


        }
    }
}