import Control.Applicative ((<$>))

main :: IO ()
main = do
  solve <$> getl (map read . words) >>= print

solve :: [Int] -> Int
solve [l, k] = let (c, r) = l `divMod` (2 * k)
               in if r == 0 then (c - 1) * k else c * k

getl :: (String -> a) -> IO a
getl f = f <$> getLine