import java.io.*;
import java.util.*;

class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = Integer.parseInt(sc.next());
    
    int sum = 0;
    for (int i = 1; i <= N; i++) {
      if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      }
    }
    
    System.out.println(sum);
  }
} import java.io.*; io.* java import java.util.*; util.* java class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = Integer.parseInt(sc.next());
    
    int sum = 0;
    for (int i = 1; i <= N; i++) {
      if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      }
    }
    
    System.out.println(sum);
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = Integer.parseInt(sc.next());
    
    int sum = 0;
    for (int i = 1; i <= N; i++) {
      if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      }
    }
    
    System.out.println(sum);
  } main {
    Scanner sc = new Scanner(System.in);
    int N = Integer.parseInt(sc.next());
    
    int sum = 0;
    for (int i = 1; i <= N; i++) {
      if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      }
    }
    
    System.out.println(sum);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = Integer.parseInt(sc.next()); N Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int sum = 0; sum 0 for (int i = 1; i <= N; i++) {
      if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      }
    } int i = 1; int i = 1; i 1 i <= N i N i++ i++ i {
      if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      }
    } {
      if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      }
    } if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      } 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 {
        sum++;
      } sum++ sum System.out.println(sum) System.out.println System.out System System.out sum String[] args args
import java.io.*;
import java.util.*;

class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = Integer.parseInt(sc.next());
    
    int sum = 0;
    for (int i = 1; i <= N; i++) {
      if (String.valueOf(i).length() % 2 == 1) {
        sum++;
      }
    }
    
    System.out.println(sum);
  }
}