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 PHP (5.6.30) (judge verdict: Accepted)

<?php
  
while ($line = fgets(STDIN)) {
   $stdin[] = trim($line);
}
$v2 = str_split($stdin[0]);
#$v2 = explode(" ", $stdin[0]);

#dream dreamer erase eraser

#var_dump($v2);

$cnt = 0;
for($i=1; $i<=$stdin[0]; $i++){

  $char = strlen($i);
  
  if($char % 2 != 0){
    $cnt++;
  }
  
}

$ans = $cnt;


echo $ans . PHP_EOL;