import java.io.BufferedReader;
import java.io.*;
import java.util.*;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Collections;
import java.util.Arrays;

public class Main {
	public static void main(final String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();

    int count = 0;

    for(int i = 1; i < n + 1; i++){

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    }

    System.out.println(count);


    
  

    


    


    

    
    
    



    





}
} import java.io.BufferedReader; BufferedReader java.io import java.io.*; io.* java import java.util.*; util.* java import java.util.ArrayList; ArrayList java.util import java.util.Iterator; Iterator java.util import java.util.List; List java.util import java.util.Collections; Collections java.util import java.util.Arrays; Arrays java.util public class Main {
	public static void main(final String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();

    int count = 0;

    for(int i = 1; i < n + 1; i++){

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    }

    System.out.println(count);


    
  

    


    


    

    
    
    



    





}
} Main public static void main(final String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();

    int count = 0;

    for(int i = 1; i < n + 1; i++){

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    }

    System.out.println(count);


    
  

    


    


    

    
    
    



    





} main {
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();

    int count = 0;

    for(int i = 1; i < n + 1; i++){

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    }

    System.out.println(count);


    
  

    


    


    

    
    
    



    





} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int count = 0; count 0 for(int i = 1; i < n + 1; i++){

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    } int i = 1; int i = 1; i 1 i < n + 1 i n + 1 n 1 i++ i++ i {

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    } {

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    } if(String.valueOf( i ).length() % 2 == 1){
            count++;
        } String.valueOf( i ).length() % 2 == 1 String.valueOf( i ).length() % 2 String.valueOf( i ).length() String.valueOf( i ).length String.valueOf( i ) String.valueOf String i 2 1 {
            count++;
        } count++ count System.out.println(count) System.out.println System.out System System.out count final String[] args args public class Main {
	public static void main(final String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();

    int count = 0;

    for(int i = 1; i < n + 1; i++){

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    }

    System.out.println(count);


    
  

    


    


    

    
    
    



    





}
} public class Main {
	public static void main(final String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();

    int count = 0;

    for(int i = 1; i < n + 1; i++){

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    }

    System.out.println(count);


    
  

    


    


    

    
    
    



    





}
} Main
import java.io.BufferedReader;
import java.io.*;
import java.util.*;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Collections;
import java.util.Arrays;

public class Main {
	public static void main(final String[] args){
    
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();

    int count = 0;

    for(int i = 1; i < n + 1; i++){

        if(String.valueOf( i ).length() % 2 == 1){
            count++;
        }
    }

    System.out.println(count);


    
  

    


    


    

    
    
    



    





}
}