import Data.List (sort)

main :: IO ()
main = interact $ solve . sort . map read . tail . words

solve :: [Int] -> String
solve xs = if n `mod` 2 == 1 
                then show x
                else show ((x + y) `div` 2) ++ "." ++ show ((((x + y) `mod` 2) * 10) `div` 2)
    where
        n  = length xs
        ys = sort xs
        x = xs !! (n `div` 2)
        y = xs !! ((n `div` 2) - 1)