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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.*;
/*being john malkovich*/
class Main
{
	public static void main(String[] args)
	{
		long max = (long)(Math.pow(10,18));
		Scanner s = new Scanner(System.in);
		int x = s.nextInt();
		int y = s.nextInt();
		if(x%y == 0 || y%x == 0)
			System.out.println("-1");
		else
		{
			int flag = 0;
			for(long i =1;i*x < max;i++)
			{
				if((i*x)%y!=0)
				{
					flag = 1;
					System.out.println(i*x);		
					break;
				}
			}
			if(flag == 0)
				System.out.println("-1");
		}
	}

}