<?php

// your code goes here
list($l, $k) = explode(' ', trim(fgets(STDIN)));

$eatin = 0;
$finish = false;
do {
	if ($eatin*2 + $k*2 >= $l) {
		$finish = true;
	} else {
		$eatin += $k;
	}
} while(!$finish);

echo $eatin;