import Data.Int
import Text.Printf

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

lmin = 10**(-9) :: Double
lmax = 10**9 :: Double

main = do
 getLine
 ls <- map (fromIntegral . (read::String->Integer)) . words <$> getLine
 k <- readLn
 printf "%.12f\n" (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))