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.*;

public class Main{
    public static void main(String[] args) {
	Scanner sc = new Scanner(System.in);
	Long X = sc.nextLong();
	Long Y = sc.nextLong();
	//System.out.println(1e18);
	if ( X % Y == 0 ) System.out.println(-1);
	else {
	    for ( int i = 2; X * i <= 1e18; i++ ) {
		X *= i;
		if ( !(X % Y == 0) ) {
		    System.out.println(X);
		    return;
		}
	    }
	    System.out.println(-1);
	}
    }
}