<?php
list($l, $k) = explode(" ", (trim(fgets(STDIN))));
$ans = 0;
if (($l/2) % $k > 0) {
    $ans = intval(($l/2) / $k) * $k;
} elseif ($k == 1) {
    $ans = (($l + 1) / 2) - 1;
}
echo $ans."\n";