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)

object Main {

  def func(a: Long, b: Long): Long = {
    var result = a * 2
    while(true) {
      if (result % b != 0)
        return result
      result += a
    }
    return -1
  }

  def main(args: Array[String]): Unit = {
    val context = Console.readLine()
    val numbers = context.split(" ").map(_.toLong)

    if (numbers(1) % numbers(0) == 0) {
      println("-1")
    } else {
      println(func(numbers(0), numbers(1)))
    }
  }
}