import           Control.Monad
import qualified Data.ByteString.Char8 as BS
import           Data.Maybe
import           Text.Printf

readInt = fst . fromJust . BS.readInt
readIntList = map readInt . BS.words
getInt = readInt <$> BS.getLine
getIntList = readIntList <$> BS.getLine

main = do
  l <- getInt
  let l' = fromIntegral l / 3.0 :: Double
      ans = l' ^ 3
  printf "%.12f" ans