<?php

/**
 * No.63 ポッキーゲーム
 * 
 * @see https://yukicoder.me/problems/134
 */

 list($pockeyLengh, $eatLength) = explode(" ", trim(fgets(STDIN)));

 $remainingLength = $pockeyLengh;
 $totalEatLengthByYu = 0;
 while (($remainingLength -= $eatLength * 2) > 0) {
    $totalEatLengthByYu += $eatLength;
 }
 
 echo $totalEatLengthByYu . "\n";