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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.IO.Pipes;
using System.Linq;
using System.Numerics;

namespace Atcoder
{
	class Program
	{
		/*
			string s = Console.ReadLine();
			string[] ST = Console.ReadLine().Split(' ').ToArray();
			string S = strArray[0];
			string T = strArray[1];

			int[] HWN = Console.ReadLine().Split(' ').Select(i => int.Parse(i)).ToArray();
			int H = intArray[0];
			int W = intArray[1];
			int N = int.Parse(Console.ReadLine());

			List<int> A = new List<int>(Console.ReadLine().Split(' ').Select(i => int.Parse(i)).ToList());
			
			double[] R = Console.ReadLine().Split(' ').Select(i => double.Parse(i)).ToArray();

			Boolean ok = false;
			
			List<int> dp = Enumerable.Repeat(値, 個数).ToList();
			
			Console.WriteLine(ans);
			Console.WriteLine("Yes");
			Console.WriteLine("No");
			Console.ReadKey();
			Console.WriteLine(cnt);

			for(int i= 0; i < N; i++)
		*/

		static void Main(string[] args)
		{
			int N = int.Parse(Console.ReadLine());
			int len = N.ToString().Length;
			
			if(len == 1)
            {
				Console.WriteLine(N);
				return;
			}
			
			int cnt = 0;
			int line = (int)Math.Pow(10, len - 1);

			if(len % 2 != 0)
            {
				cnt += N - line + 1;
            }
			int num = 9;
			for(int i = 1;i < line;i *= 100)
            {
				cnt += num;
				num *= 100;
            }
			Console.WriteLine(cnt);
		}

		class Item : IComparable
		{
			public string A;
			public int B;
			public Item(string a, int b)
			{
				this.A = a;
				this.B = b;
			}
			public int CompareTo(object obj)
			{
				return B - ((Item)obj).B;
			}
		}

		/// <summary>
		/// 回文
		/// </summary>
		/// <param name="Target"></param>
		/// <param name="len"></param>
		/// <returns></returns>
		static bool Palindrome(string Target , int len)
        {
			bool yes = true;

			for(int i = 0;i < len; i++)
            {
				if(Target[i] != Target[len - 1 - i])
                {
					yes = false;
					break;
                }
            }
			return yes;
        }

		/// <summary>
		/// アナグラム
		/// </summary>
		/// <param name="word1"></param>
		/// <param name="word2"></param>
		/// <returns></returns>
		static bool IsAnagramOf(string word1, string word2)
		{
			char[] val1 = word1.ToCharArray();
			char[] val2 = word2.ToCharArray();
			Array.Sort(val1);
			Array.Sort(val2);
			word1 = new string(val1);
			word2 = new string(val2);

			return word1 == word2;
			//return word1.OrderBy(x => x).SequenceEqual(word2.OrderBy(x => x));
		}

		/// <summary>
		/// 最小公倍数
		/// </summary>
		/// <param name="a"></param>
		/// <param name="b"></param>
		/// <returns></returns>
		public static BigInteger Lcm(BigInteger a, BigInteger b)
		{
			return a * b / Gcd(a, b);
		}

		/// <summary>
		/// 最大公約数
		/// </summary>
		/// <param name="a"></param>
		/// <param name="b"></param>
		/// <returns></returns>
		public static BigInteger Gcd(BigInteger a, BigInteger b)
		{
			if (a < b)
				// 引数を入替えて自分を呼び出す
				return Gcd(b, a);
			while (b != 0)
			{
				var remainder = a % b;
				a = b;
				b = remainder;
			}
			return a;
		}
	}
}