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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;

class Myclass
{
    static void Main()
    {
        int N = int.Parse(Console.ReadLine());
        int ans=0;
        if (N == 100000)
        {
            N = 99999;
        }
        if (N >= 10000)
        {
            ans += N - 9999;
            N = 9999;
        }
        if (N >= 1000)
        {
            N = 999;
        }
        if (N >= 100)
        {
            ans += N - 99;
            N = 99;
        }
        if (N >= 10)
            N = 9;
        ans += N;
        Console.WriteLine(ans);
            
    }
}