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.Text;
using System.Threading.Tasks;

namespace VS_atcoder
{

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



            int N = Int32.Parse(Console.ReadLine());
            String preX = Console.ReadLine();
            String preY = Console.ReadLine();

            //int[] after = before.Select(s => int.Parse(s)).ToArray();
            var a = preX.Split(' ').Select(s => int.Parse(s)).ToArray();
            var b = preY.Split(' ').Select(s => int.Parse(s)).ToArray();
            int yoryoku = 0;
            int minsum = 0;
            for (int i = 0; i < N; i++)
            {
                if (a[i] > b[i])
                {
                    minsum += a[i] - b[i];
                }
                else if (b[i] > a[i])
                {
                    yoryoku += b[i] - a[i];
                }


            }

            //余力が最低加算を上回っていればOK
            if (yoryoku/2 >= minsum)
            {
                Console.WriteLine("Yes");
            }
            else
            {
                Console.WriteLine("No");
            }

            return;

        }


    }
}