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.Scanner;

public class Main {

  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    long X = scanner.nextLong();
    long Y = scanner.nextLong();

    new Main().execute(X, Y);
  }

  public void execute(long X, long Y) {
    long MAX = (long)Math.pow(10, 18);
    if (X % Y == 0) {
      System.out.println("−1");
      return;
    }

    for (long i = 1; i < Y - 1; i++) {
      long multiple = X * i;
      if (multiple % Y != 0 && multiple <= MAX) {
        System.out.println(multiple);
        return;
      }
    }

    System.out.println("−1");
  }

}