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 Scala (2.11.7) (judge verdict: Wrong Answer)

import java.util.Scanner

/**
  * Created by 80011714 on 2018/2/5.
  */
object Main {

  def solve(sc: Scanner) = {
    val x, y = sc.nextLong()

    println(if (x % y == 0) (-1) else {
      var i = 1
      while (i * x % y == 0) (i += 1)
      i * x
    })
  }

  def main(args: Array[String]): Unit = {
    val sc: Scanner = new Scanner(System.in)
  }

}