import Data.Ratio
import Text.Printf

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

lmin = 0
lmax = 10e9
nbs = 50

main = do
 getLine
 ls <- map (fromIntegral . read) . words <$> getLine
 k <- readLn
 printf "%.12f" (yokuaru k ls)

yokuaru :: Integer -> [Ratio Integer] -> Double
yokuaru k ls = fromRational (fst (apply nbs (bsearch k ls) (1,(lmin,lmax))))

bsearch k ls (x,(xn,xx))
 | cuts x ls < k = (x - (x-xn) / 2, (xn,x))
 | otherwise = (x + (xx-x) / 2, (x,xx))

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