import 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);

	}
} 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 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 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.*;

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);

	}
}