import java.util.Scanner;
public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int seisu = scan.nextInt();
		int count = 0;

		for(int i=1;i<=seisu;i++) {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();

	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int seisu = scan.nextInt();
		int count = 0;

		for(int i=1;i<=seisu;i++) {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();

	}

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

		Scanner scan = new Scanner(System.in);

		int seisu = scan.nextInt();
		int count = 0;

		for(int i=1;i<=seisu;i++) {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();

	} main {

		Scanner scan = new Scanner(System.in);

		int seisu = scan.nextInt();
		int count = 0;

		for(int i=1;i<=seisu;i++) {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();

	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int seisu = scan.nextInt(); seisu scan.nextInt() scan.nextInt scan int count = 0; count 0 for(int i=1;i<=seisu;i++) {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		} int i=1; int i=1; i 1 i<=seisu i seisu i++ i++ i {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		} {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		} String mozi = String.valueOf(i); mozi String.valueOf(i) String.valueOf String i if(mozi.length()%2 == 1) {
				count++;
			} mozi.length()%2 == 1 mozi.length()%2 mozi.length() mozi.length mozi 2 1 {
				count++;
			} count++ count System.out.println(count) System.out.println System.out System System.out count scan.close() scan.close scan String[] args args public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int seisu = scan.nextInt();
		int count = 0;

		for(int i=1;i<=seisu;i++) {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();

	}

} public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int seisu = scan.nextInt();
		int count = 0;

		for(int i=1;i<=seisu;i++) {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();

	}

} Main
import java.util.Scanner;
public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);

		int seisu = scan.nextInt();
		int count = 0;

		for(int i=1;i<=seisu;i++) {
			String mozi = String.valueOf(i);
			if(mozi.length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();

	}

}
