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)

import Data.Bits
import Data.List
import Control.Monad
import qualified Data.Vector.Unboxed as V
import qualified Data.Vector.Unboxed.Mutable as VM
import Data.Array
import Data.Array.MArray
import Data.Array.ST
import Control.Monad.ST
import Data.Maybe (fromMaybe)
import qualified Data.ByteString.Lazy.Char8 as B
import Debug.Trace


main :: IO ()
main = do
  n <- getLine
  let l = length n
  (print :: Int -> IO ()) $ if l == 1 then read n else (+) (if odd l then (read n) - 10^(l-1) + 1 else 0) $ sum [10^(l'-1)*9 | l' <- [1,3..if odd l then l-2 else l-1]]