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.Linq;

namespace codeTester.contest190804
{
    class ProblemB
    {
        static int n = 0;
        static string answer = "";

        public static void Main(string[] args)
        {
            Input();
            Logic();
            Output();
        }

        static void Input()
        {
            n = int.Parse(Console.ReadLine());
        }

        static void Logic()
        {
            int inputDigitNum = n.ToString().Length;
            switch (inputDigitNum)
            {
                case 6:
                    answer = 90909.ToString();
                    break;
                case 5:
                    answer = (n - 9999 + 909).ToString();
                    break;
                case 4:
                    answer = 909.ToString();
                    break;
                case 3:
                    answer = (n - 99 + 9).ToString();
                    break;
                case 2:
                    answer = 9.ToString();
                    break;
                case 1:
                    answer = n.ToString();
                    break;
            }
        }

        static void Output()
        {
            Console.WriteLine(answer);
        }

        static void DebugPrint(string msg)
        {
            // 提出時に消す
            // Console.WriteLine(msg);
        }
    }
}