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", $n);
$n1 = fgets(STDIN);
$n2 = fgets(STDIN);

$a = explode(' ', $n1);
$b = explode(' ', $n2);

$aSum = 0;
$bSum = 0;
$distance = 0;
foreach ($a as $index => $ai) {
    $bi = $b[$index];
    $aSum += $ai;
    $bSum += $bi;
    if ($ai >= $bi) {
        $distance += $ai - $bi;
        continue;
    }
    $distance = floor(($bi - $ai) / 2) + ceil(($bi - $ai) % 2);
}

$sumDiff = $bSum - $aSum;
if ($sumDiff >= $distance) {
    echo "Yes";
    return;
}

echo "No";
return;