import java.util.*;

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

    int n = sc.nextInt();
    int ans = 0;

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

    System.out.println(ans);
  }
} import java.util.*; util.* java public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    int ans = 0;

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

    System.out.println(ans);
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    int ans = 0;

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

    System.out.println(ans);
  } main {
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    int ans = 0;

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

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

    int n = sc.nextInt();
    int ans = 0;

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

    System.out.println(ans);
  }
} public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    int ans = 0;

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

    System.out.println(ans);
  }
} Main
import java.util.*;

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

    int n = sc.nextInt();
    int ans = 0;

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

    System.out.println(ans);
  }
}
