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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
  fscanf(STDIN, "%d", $N);
  $ans = 0;
  $x = min(9, $N);
  $ans += $x;
  if ($N >= 100) {
    $y = min(999, $N);
    $ans += $y - 99;
  }
  if ($N >= 10000) {
    $z = min(99999, $N);
    $ans += $z - 9999;
  }
  echo $ans;