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: Accepted)

(defun solve (N)
  (let ((digits (length (write-to-string N))))
    (cond
     ((= digits 5) (- N 9090))
     ((= digits 4) 909)
     ((= digits 3) (- N 90) )
     ((= digits 2) 9)
     ((= digits 1) N)
     (t 90909))
    ))
(princ (solve (read)))