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: Time Limit Exceeded)

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner stdin = new Scanner(System.in);

		String line = stdin.nextLine();
		stdin.close();
		String[] input = line.split(" ");
		int num = Integer.parseInt(input[0]);
		int not = Integer.parseInt(input[1]);

		if(num == not || (num % 2 == 0 && not  == 2) || not == 1 || not == 0){
			System.out.println(-1);
			return;
		}else if(num < not){
			System.out.println(num);
			return;
		}
		for(long i = 0; i < 1000000000000000000l; i += num){
			if(i % not != 0){
				System.out.println(i);
				return;
			}
		}
		System.out.println(-1);
	}

}