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;
using System.Text;
using System.Threading.Tasks;

namespace VS_atcoder
{

    class Program
    {
        static void Main(string[] args)
        {


            String str = Console.ReadLine();

            int X = Int32.Parse(str.Split(' ')[0]);
            int Y = Int32.Parse(str.Split(' ')[1]);

            long minXY = X > Y ? X : Y;
            long max = (long)Math.Pow(10, 18);
            int min = Y > X ? Y / X : 2;

            for (int i = min > 1? min : 2; i < max; i++)
            {
                if (Y == 1 || X == Y || X % Y == 0) break;
                if ((X * i) % Y != 0)
                {
                    Console.WriteLine(X * i);
                    return;
                }



                if ((X * i) > max) break;
            }




            Console.WriteLine(-1);
            return;

        }


    }
}