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;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            int n = int.Parse(input);

            int[] ary1 = new int[n];
            int[] ary2 = new int[n];

            input = Console.ReadLine();
            string[] words = input.Split(' ');
            for (int i = 0; i < n; i++)
            {
                ary1[i] = int.Parse(words[i]);
            }

            input = Console.ReadLine();
            words = input.Split(' ');
            for (int i = 0; i < n; i++)
            {
                ary2[i] = int.Parse(words[i]);
            }

            int sum = 0;
            for (int i = 0; i < n; i++)
            {
                sum = sum + (ary1[i] - ary2[i]);
            }

            if (System.Math.Abs(sum)% 2 == 0)
            {
                Console.WriteLine("No");
            }else
            {
                Console.WriteLine("Yes");
            }


        }
        }
}