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 sc = new Scanner(System.in);
		// 要素の入力、格納
		int X = sc.nextInt();
		int Y = sc.nextInt();
		long ans = -1;
		long max = (long) Math.pow(10, 18);

		if ((Y / X > 1) && (Y % X == 0)) {
		} else if (X == Y) {
		} else {
			for (int i = 1; i <= max / X; i++) {
				if ((X * i) % Y != 0) {
					ans = X * i;
					break;
				}
			}
		}
		// 出力
		System.out.println(ans);
	}
}