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 Java7 (OpenJDK 1.7.0) (judge verdict: Time Limit Exceeded)

import java.util.Scanner;
public class Main

{
    public static void main(String arg[])
    {
        int a,b;
        Scanner scan=new Scanner(System.in);
        a=scan.nextInt();
        b=scan.nextInt();
        if(a==b)
        System.out.println("-1");
        int c=a>b?a:b;
        while(2>1)
        {
            if(c%a==0&&c%b!=0&&c!=a)
            {
            System.out.println(c);
            break;
            }
            c=c+a;
        }
    }
}