module Main where

type Input  = Double
type Output = Double

parse :: String -> Input
parse = read . head . lines

solve :: Input -> Output
solve n =
    let
        w = n / 3

        h = w / 2 * sqrt 3
    in
        w * h / 2

render :: Output -> String
render = show

main :: IO ()
main =
    interact
        ( render
        . solve
        . parse
        )