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.List
import Data.Char
import Data.Maybe
import qualified Data.ByteString.Char8 as BS8

dig_impl 0 r = r
dig_impl n r = dig_impl (div n 10) (r + 1)

dig n = dig_impl n 0

cnt n r
    | dig n == 1 = n
    | dig n == 2 = 9
    | dig n == 3 = n - 90
    | dig n == 4 = 909
    | dig n == 5 = n - 9090
    | dig n == 6 = 90909

main :: IO ()
main = do
    n <- fst . fromJust . BS8.readInteger <$> BS8.getLine

    print $ cnt n n