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.Collections.Generic;
using System.Text;
using System.Numerics;
using System.IO;


namespace SolutionCS
{
    class Program
    {
        static void Main(string[] args)
        {
            checked
            {
                //string[] s;
#if true
                int N = int.Parse(Console.ReadLine());
#else
                s = Console.ReadLine().Split(' ');
                int A = int.Parse(s[0]);
                int B = int.Parse(s[1]);
                int C = int.Parse(s[2]);
#endif


#if true
                /*
                s = Console.ReadLine().Split(' ');
                for (int i = 0; i < N; i++)
                {
                    v[i] = int.Parse(s[i]);
                }
                */
#else
                for (int i = 0; i < N; i++)
                {
                    v[i] = int.Parse(Console.ReadLine());
                }
#endif
                int ans = 0;
                for (int i = 1; i <= N; i++)
                {
                    if ( i <= 9)
                    {
                        ans++;
                    }
                    else if( i >= 100 & i <= 999)
                    {
                        ans++;
                    }
                    else if (i >= 10000 & i <= 99999)
                    {
                        ans++;
                    }
                }


                Console.WriteLine(ans);
            }
        }
    }
}