import java.util.Scanner;

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

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

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

	  System.out.println(count);
  } main {
	  Scanner sc = new Scanner(System.in);
	  int n=sc.nextInt(),count=0;
	  String s="";
	  for(int i=1;i<=n;i++) {
		  s=String.valueOf(i);
		  if(s.length()%2!=0)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 count=0; count 0 String s=""; s "" for(int i=1;i<=n;i++) {
		  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 {
		  s=String.valueOf(i);
		  if(s.length()%2!=0)count++;
	  } {
		  s=String.valueOf(i);
		  if(s.length()%2!=0)count++;
	  } 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 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 sc = new Scanner(System.in);
	  int n=sc.nextInt(),count=0;
	  String s="";
	  for(int i=1;i<=n;i++) {
		  s=String.valueOf(i);
		  if(s.length()%2!=0)count++;
	  }

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

	  System.out.println(count);
  }
} Main
import java.util.Scanner;

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

	  System.out.println(count);
  }
}
