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 scala.io.StdIn._

object Main {
    def main(args: Array[String]): Unit = {
        val Array(x, y) = readLine.split(" ").map(_.toInt)
        if (y % x == 0) {
            println(-1)
        } else {
            println(f(x, y, 1))
        }
    }
    
    def f(x: Int, y: Int, n: Int): Int = {
        if (x * n >= Math.pow(10, 18) ) {
            -1
        } else if (x * n % y == 0) {
            f(x, y, n + 1)
        } else {
            x * n
        }
    }
}