import qualified Data.Text as T
import Control.Applicative
main = do
	inputs<-fmap (read.T.unpack)<$>T.split (==' ')<$>T.pack<$>getLine::IO [Int]
	let l=inputs!!0
	let k=inputs!!1
	let n=l `div` (2*k)
	let n1=if l `mod` (2*k)==0 then n-1 else n
	print$n1*k