import Data.Fixed
import Text.Printf
import Data.Maybe
import qualified Data.ByteString.Char8 as B

apply n f x = foldr ($) x (replicate n f)

lmin = 1e-9 :: Pico
lmax = 1e9 :: Pico
eps = 1e-10 :: Pico

main = do
 getLine
 ls <- map (fromIntegral . fst . fromJust . B.readInteger) . B.words <$> B.getLine :: IO [Pico]
 k <- readLn
 putStrLn $ showFixed True (yokuaru k ls)

yokuaru k ls = snd $ apply 60 (bsearch k ls) (lmin,lmax)

bsearch k ls (lo,hi)
 | cuts md ls < k = (lo,md)
 | otherwise = (md,hi)
 where
  md = (lo + hi) / 2

cuts x = sum . map (floor . (/x))