{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE NegativeLiterals #-}
{-# LANGUAGE ParallelListComp #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE NPlusKPatterns #-}
{-# LANGUAGE StrictData #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE ViewPatterns #-}

import           Control.Applicative
import           Control.Arrow
import           Control.Monad
-- import           Control.Monad.Extra
import           Control.Monad.Fix
import           Control.Monad.ST
import           Data.Bits
-- import qualified Data.Vector.Unboxed.Base      as UB
-- import qualified Data.Vector.Generic.Mutable.Base
--                                                as MB
-- import qualified Data.Vector.Generic.Base      as GB
import qualified Data.ByteString.Char8         as BS
import           Data.Char
import           Data.Foldable
import           Data.Functor
import           Data.Int
import           Data.List
import           Data.Maybe
import           Data.Monoid
import           Data.Ord
import           Data.Semigroup                 ( Max(..)
                                                , Min(..)
                                                , All(..)
                                                , Arg(..)
                                                )
import           Data.Ratio
-- import           Data.Vector.Unboxing.Mutable   ( Unboxable )
-- import qualified Data.Vector.Unboxing.Mutable  as V
import           System.IO
import           Data.Proxy
import           GHC.TypeLits
import           GHC.Generics                   ( Generic )
import           GHC.Exts


{-# ANN module ("Hlint: ignore Unused LANGUAGE pragma" :: String) #-}


twice :: (a -> a -> b) -> a -> b
twice f x = f x x

both :: Arrow a => a b c -> a (b, b) (c, c)
both = twice (***)

readsLn :: Read a => IO [a]
readsLn = mapM readIO . words =<< getLine

getInt :: IO Int
getInt = fst . fromJust . BS.readInt <$> BS.getLine

getInts :: IO [Int]
getInts = map (fst . fromJust . BS.readInt) . BS.words <$> BS.getLine

getInteger :: IO Integer
getInteger = fst . fromJust . BS.readInteger <$> BS.getLine

getIntegers :: IO [Integer]
getIntegers = map (fst . fromJust . BS.readInteger) . BS.words <$> BS.getLine


main = do
  k <- readLn @Double
  print if k == 0 then pi * pi / 6 else sum (map recip [1 .. k]) / k