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 = interact $ show . process . read
process n 
	| even m = b $ m 
    | otherwise = ( a n m ) + ( b $ m - 1 )
	where
    	m = k n
    	k 0 = 0
        k t = 1 + (k $ div t 10)
        a n m = n + 1 - (10 ^ (m - 1))
        b x = div ( 10 ^ x - 1 ) 11