import java.util.Scanner;

public class Main {

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

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

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