import Control.Monad

rInt :: String -> Int
rInt = read

pokkyGame l k
    | l <= k * 2    = 0     -- 一度も齧れない
    | otherwise     = k * kaisuu
    where
        kaisuu = l `div` (k * 2)

main = do
    lk <- map rInt . words <$> getLine
    print $ pokkyGame (lk !! 0) (lk !! 1)