import java.util.*;

public class Main {

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int count = 0;
    for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    }
    System.out.println(count);
  }



}

 import java.util.*; util.* java public class Main {

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int count = 0;
    for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    }
    System.out.println(count);
  }



} Main public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int count = 0;
    for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    }
    System.out.println(count);
  } main {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int count = 0;
    for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    }
    System.out.println(count);
  } Scanner input = new Scanner(System.in); input new Scanner(System.in) int n = input.nextInt(); n input.nextInt() input.nextInt input int count = 0; count 0 for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    } int i = 1; int i = 1; i 1 i<=n i n i++ i++ i {
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    } {
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    } String s = String.valueOf(i); s String.valueOf(i) String.valueOf String i if(s.length() % 2 != 0){
        count++;
      } s.length() % 2 != 0 s.length() % 2 s.length() s.length s 2 0 {
        count++;
      } count++ count System.out.println(count) System.out.println System.out System System.out count String[] args args public class Main {

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int count = 0;
    for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    }
    System.out.println(count);
  }



} public class Main {

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int count = 0;
    for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    }
    System.out.println(count);
  }



} Main
import java.util.*;

public class Main {

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int count = 0;
    for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    }
    System.out.println(count);
  }



}


