using System;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Collections.Generic;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using static System.Math;
using Debug = System.Diagnostics.Debug;
using MethodImplOptions = System.Runtime.CompilerServices.MethodImplOptions;
using MethodImplAttribute = System.Runtime.CompilerServices.MethodImplAttribute;

static class P
{
    static void Main()
    {
        int count = 0;
        int n = int.Parse(Console.ReadLine());
        for (int i = 1; i <= n; i++)
        {
            if (i.ToString().Length % 2 == 1) count++;
        }
        Console.WriteLine(count);
    }



    static readonly TextReader In = Console.In;
    static int NextInt
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        get
        {
            int res = 0;
            int next = In.Read();
            int rev = 1;
            while (45 > next || next > 57) next = In.Read();
            if (next == 45) { next = In.Read(); rev = -1; }
            while (48 <= next && next <= 57)
            {
                res = res * 10 + next - 48;
                next = In.Read();
            }
            return res * rev;
        }
    }
}


class ReRooting<T>
{
    public int NodeCount { get; private set; }

    int[][] Neighbours;
    int[][] IndexForNeighbours;

    T[] Res;
    T[][] dp;

    T IdentityElement;
    Func<T, T, T> Operate;
    Func<T, T> OperateNode;

    public ReRooting(int nodeCount, int[][] edges, T identityElement, Func<T, T, T> operate, Func<T, T> operateNode)
    {
        NodeCount = nodeCount;

        IdentityElement = identityElement;
        Operate = operate;
        OperateNode = operateNode;

        List<int>[] neighbours = new List<int>[nodeCount];
        List<int>[] indexForNeighbours = new List<int>[nodeCount];

        for (int i = 0; i < nodeCount; i++)
        {
            neighbours[i] = new List<int>();
            indexForNeighbours[i] = new List<int>();
        }
        for (int i = 0; i < edges.Length; i++)
        {
            var edge = edges[i];
            indexForNeighbours[edge[0]].Add(neighbours[edge[1]].Count);
            indexForNeighbours[edge[1]].Add(neighbours[edge[0]].Count);
            neighbours[edge[0]].Add(edge[1]);
            neighbours[edge[1]].Add(edge[0]);
        }

        Neighbours = new int[nodeCount][];
        IndexForNeighbours = new int[nodeCount][];
        for (int i = 0; i < nodeCount; i++)
        {
            Neighbours[i] = neighbours[i].ToArray();
            IndexForNeighbours[i] = indexForNeighbours[i].ToArray();
        }

        dp = new T[Neighbours.Length][];
        Res = new T[Neighbours.Length];

        for (int i = 0; i < Neighbours.Length; i++) dp[i] = new T[Neighbours[i].Length];
        if (NodeCount > 1) Initialize();
        else Res[0] = OperateNode(IdentityElement);
    }

    public T Query(int node) => Res[node];

    private void Initialize()
    {
        int[] parents = new int[NodeCount];
        int[] order = new int[NodeCount];

        #region InitOrderedTree
        var index = 0;
        Stack<int> stack = new Stack<int>();
        stack.Push(0);
        parents[0] = -1;
        while (stack.Count > 0)
        {
            var node = stack.Pop();
            order[index++] = node;
            for (int i = 0; i < Neighbours[node].Length; i++)
            {
                var neighbour = Neighbours[node][i];
                if (neighbour == parents[node]) continue;
                stack.Push(neighbour);
                parents[neighbour] = node;
            }
        }
        #endregion

        #region fromLeaf
        for (int i = order.Length - 1; i >= 1; i--)
        {
            var node = order[i];
            var parent = parents[node];

            T accum = IdentityElement;
            int neighbourIndex = -1;
            for (int j = 0; j < Neighbours[node].Length; j++)
            {
                if (Neighbours[node][j] == parent)
                {
                    neighbourIndex = j;
                    continue;
                }
                accum = Operate(accum, dp[node][j]);
            }
            dp[parent][IndexForNeighbours[node][neighbourIndex]] = OperateNode(accum);
        }
        #endregion

        #region toLeaf
        for (int i = 0; i < order.Length; i++)
        {
            var node = order[i];
            T accum = IdentityElement;
            T[] accumsFromTail = new T[Neighbours[node].Length];
            accumsFromTail[accumsFromTail.Length - 1] = IdentityElement;
            for (int j = accumsFromTail.Length - 1; j >= 1; j--) accumsFromTail[j - 1] = Operate(dp[node][j], accumsFromTail[j]);
            for (int j = 0; j < accumsFromTail.Length; j++)
            {
                dp[Neighbours[node][j]][IndexForNeighbours[node][j]] = OperateNode(Operate(accum, accumsFromTail[j]));
                accum = Operate(accum, dp[node][j]);
            }
            Res[node] = OperateNode(accum);
        }
        #endregion
    }
}


Measured memory_bytes: 13140