Problem:
Score : 200 points

Problem Statement

Given is an integer N. Find the number of positive integers less than or equal to N that have an odd number of digits (in base ten without leading zeros).

Constraints

1 \leq N \leq 10^5

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of positive integers less than or equal to N that have an odd number of digits.

Sample Input 1

11

Sample Output 1

9

Among the positive integers less than or equal to 11, nine integers have an odd number of digits: 1, 2, \ldots, 9.

Sample Input 2

136

Sample Output 2

46

In addition to 1, 2, \ldots, 9, another 37 integers also have an odd number of digits: 100, 101, \ldots, 136.

Sample Input 3

100000

Sample Output 3

90909

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

-- ABC 136 B
import Data.Maybe
import Data.Char
import Data.List
import qualified Data.ByteString.Char8 as BS
import qualified Data.Vector.Unboxed as VU

main :: IO()
main = do
  n <- getInt
  print $ solve n

solve :: Int -> Int
solve n = if odd rank then m + l else l
  where
    n' = show n
    rank = length n'
    m = n - 10^(rank-1) + 1
    l = sum . map (\k -> 9*10^(k-1)) $ [1,3..rank-1]

-- ByteStringを用いた入力
readInt = fst . fromJust . BS.readInt

-- 整数を一つ受け取る
getInt = readInt <$> BS.getLine

-- 長さnの整数列を受け取る
getIntVec n = VU.unfoldrN n (BS.readInt . BS.dropWhile isSpace) <$> BS.getLine

-- 空白区切りの整数列をListで受け取る
getIntList = unfoldr (BS.readInt . BS.dropWhile isSpace) <$> BS.getLine