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)

main = do
  li <- getLine
  let n = read li
  let ans = compute n
  print ans

compute :: Int -> Int
compute n
 | n < 10 = n
 | n < 100 = 9
 | n < 1000 = 9 + n - 99
 | n < 10000 = 9 + 900
 | n < 100000 = 9 + 900 + n - 9999
 | otherwise = 9 + 900 + 90000