import Control.Monad

rInt :: String -> Int
rInt = read

pokkyGame l k
    | l <= k2           = 0     -- 一度も齧れない
    | k == 1            = l `div` 2 - 1
    | l `mod` k2 == 0   = k * (kaisuu - 1)
    | otherwise         = k * kaisuu
    where
        k2 = k * 2
        kaisuu = l `div` k2

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