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);
		sc.close();

	}
} 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);
		sc.close();

	}
} 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);
		sc.close();

	} 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);
		sc.close();

	} 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 sc.close() sc.close sc 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);
		sc.close();

	}
} 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);
		sc.close();

	}
} 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);
		sc.close();

	}
}