import java.util.Scanner;

public class Main {
	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();
		int count = 0;
		
		for (int i = 1; i <= N; i++) {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (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 scan = new Scanner(System.in);

		int N = scan.nextInt();
		int count = 0;
		
		for (int i = 1; i <= N; i++) {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		}

		System.out.println(count);
	}
} Main public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();
		int count = 0;
		
		for (int i = 1; i <= N; i++) {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		}

		System.out.println(count);
	} main {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();
		int count = 0;
		
		for (int i = 1; i <= N; i++) {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		}

		System.out.println(count);
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int N = scan.nextInt(); N scan.nextInt() scan.nextInt scan int count = 0; count 0 for (int i = 1; i <= N; i++) {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		} int i = 1; int i = 1; i 1 i <= N i N i++ i++ i {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		} {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		} String str = String.valueOf(i); str String.valueOf(i) String.valueOf String i int length = str.length(); length str.length() str.length str if (length % 2 == 1) {
				count++;
			} length % 2 == 1 length % 2 length 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 scan = new Scanner(System.in);

		int N = scan.nextInt();
		int count = 0;
		
		for (int i = 1; i <= N; i++) {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		}

		System.out.println(count);
	}
} public class Main {
	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();
		int count = 0;
		
		for (int i = 1; i <= N; i++) {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		}

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

public class Main {
	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();
		int count = 0;
		
		for (int i = 1; i <= N; i++) {
			String str = String.valueOf(i);
			int length = str.length();
			
			if (length % 2 == 1) {
				count++;
			}
		}

		System.out.println(count);
	}
}