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;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            string[] words = input.Split(' ');
            int n1 = int.Parse(words[0]);
            int n2 = int.Parse(words[1]);

            if (n1 % n2 == 0)
            {
                Console.WriteLine(-1);
                return;
            }

            int i = 1;
            bool flg = false;
            int num = 0;

            while(num < 1000000000000000000)
            {
                num = n1 * i;
                if (num % n2 != 0)
                {
                    Console.WriteLine(num);
                    flg = true;
                    break;
                }
                i++;
            }

            if (flg == false) {
                Console.WriteLine(-1);
            }
            
        }
        }
}