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: Wrong Answer)

<?php
# 数値の入力
fscanf(STDIN, "%d", $data1);
$anser = 0;
if($data1<10){
    $anser = $data1;
}elseif($data1<100){
    $anser = 9;
}elseif($data1<1000){
    $anser = 9+$data1-99;
}elseif($data1<10000){
    $anser = 9+900;
}elseif($data1<100000){
    $anser = 9+900+$data1-9999;
}
echo $anser."\n";
?>