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

namespace cs
{
    public class Program
    {
        public static void Main(string[] args)
        {
            /* 
            var N = long.Parse(Console.ReadLine());
            var cities = Array.ConvertAll(Console.ReadLine().Split(' '), long.Parse);
            var brave = Array.ConvertAll(Console.ReadLine().Split(' '), long.Parse);
            */

            //var abc = Array.ConvertAll(Console.ReadLine().Split(' '), long.Parse);

            //Console.WriteLine(outer);
            SLV(long.Parse(Console.ReadLine()));
            //SLV(abc);
            Console.ReadKey();
        }

        public static void SLV(long N){


            var cnt = 0;

            for(long i = 1; i <= N; i++){
                if(i >= 10 && i <= 99) continue;
                if(i >= 1000 && i <= 9999) continue;
                if(i >= 100000 && i <= 999999) continue;
                cnt++;
                 
            }

            Console.WriteLine(cnt);

            return;
        }




        public static long modder(){
            return (long)Math.Pow(10,9) + 7;
        }

        public static long GCD(long n, long m){
            
            if(m % n == 0) return n;
            else return GCD(m%n, n);

        }

        public static long LCD(long n, long m){

            return m * n / GCD(n,m);
        }

        
        // いやなんか違うけどねこれ
        public static long lower_bound(long[] array, long input){
            var left = 0;
            var right = array.Length-1;
            var mid =0;
            while(right >= left){
                mid = left + (right - left)/2;
                if(array[mid] == input){
                    return mid;
                }
                if(array[mid] > input){
                    right = mid-1;
                }else{
                    left = mid+1;
                }
            }
    
            return right;
        }

        
        class UfTree{
            public long[] parents{get;set;}
            public long[] size{get;set;} //sizeはrootのみが最新のものを保持する
    
            public UfTree(long n){
                parents = new long[n];
                size = new long[n];
                for(long i = 0; i < n; i++){
                    parents[i] = i;
                    size[i] = 1;
                } 
            }
    
            public long getRoot(long x){
                if(parents[x] == x){
                    return x;
                }else{
                    parents[x] = getRoot(parents[x]);
                    return parents[x];
                }
            }
    
            public long unite(long x, long y){
                var rx = getRoot(x);
                var ry = getRoot(y);
                if(rx == ry){
                    return rx;
                } else{
                    var sx = getSize(rx);
                    var sy = getSize(ry);
                    if(sx > sy){
                        size[rx] = sx+sy;
                        parents[ry] = rx;
                        return rx;
                    }else{
                        size[ry] = sx+sy;
                        parents[rx] = ry;
                        return ry;
                    }
                }
                
            }
    
            public long getSize(long x){
                return size[getRoot(x)];
            }
    
            
    
            public bool isSameTree(long x, long y){
                return getRoot(x) == getRoot(y);
            }
 
        }
    }


    
}


Measured memory_bytes: 13140