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 Common Lisp (SBCL 1.1.14) (judge verdict: Wrong Answer)

(let* ((n (read)))
  (cond ((<= 1 n 9) (princ n))
        ((<= 10 n 99) (princ 9))
        ((<= 100 n 999) (princ (+ 9 (- n 99))))
        ((<= 1000 n 9999) (princ 909))
        ((<= 10000 n 99999) (princ (+ 909 (- n 999))))
        (t (princ 90909))))