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 PHP7 (7.0.15) (judge verdict: Wrong Answer)

<?php

fscanf(STDIN, "%d %d", $X, $Y);

$max = 1000000000;
$limit = $max * $max;

if(1 <= $X && $X <= $max && 1 <= $Y && $Y <= $max) {
    for($i = 2; ; $i++) {
        $s = $X * $i;

        if($s % $Y != 0) {
            echo $s;
            break;
        }
        else {
            if($s <= $limit) {
                echo -1;
                break;
            }
        }
    }
}
else {
    echo -1;
}

?>