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);

if ($x >= $y) {
    $big = $x;
    $small = $y;
} else {
    $big = $y;
    $small = $x;
}

if ($big % $small === 0) {
    echo -1;
} else {
    $gcd = gcd($big, $small);
    
    $a = $big / $gcd;
    $b = $small / $gcd;
    
    $c = 2;
    
    if ($big * $c > pow(10, 18)) {
        echo -1;
    } else {
        echo $big * $c;
    }
}

function gcd($a, $b)
{
    if ($a == 0 || $b == 0)
        return abs( max(abs($a), abs($b)) );
        
        $r = $a % $b;
        return ($r != 0) ?
        gcd($b, $r) :
        abs($b);
}