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.math.BigInteger;
import java.util.Scanner;

/**
 * Created by hkoba on 2018/01/08.
 */
public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        // スペース区切りの整数の入力
        BigInteger x = BigInteger.valueOf(sc.nextInt());
        BigInteger y = BigInteger.valueOf(sc.nextInt());
        if (x.mod(y).signum() == 0) {
            System.out.println("-1");
            return;
        }
        BigInteger g = x.gcd(y);
        System.out.println(x.multiply(g));
    }
}