import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.next());
		sc.close();

		int count = 0;
		for(int i = 1; i <= N; i++) {
			int numLen = String.valueOf(i).length();
			if(numLen % 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 = Integer.parseInt(sc.next());
		sc.close();

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

		int count = 0;
		for(int i = 1; i <= N; i++) {
			int numLen = String.valueOf(i).length();
			if(numLen % 2 == 1) {
				count++;
			}
		}
		System.out.println(count);
	} main {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.next());
		sc.close();

		int count = 0;
		for(int i = 1; i <= N; i++) {
			int numLen = String.valueOf(i).length();
			if(numLen % 2 == 1) {
				count++;
			}
		}
		System.out.println(count);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = Integer.parseInt(sc.next()); N Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc sc.close() sc.close sc int count = 0; count 0 for(int i = 1; i <= N; i++) {
			int numLen = String.valueOf(i).length();
			if(numLen % 2 == 1) {
				count++;
			}
		} int i = 1; int i = 1; i 1 i <= N i N i++ i++ i {
			int numLen = String.valueOf(i).length();
			if(numLen % 2 == 1) {
				count++;
			}
		} {
			int numLen = String.valueOf(i).length();
			if(numLen % 2 == 1) {
				count++;
			}
		} int numLen = String.valueOf(i).length(); numLen String.valueOf(i).length() String.valueOf(i).length String.valueOf(i) String.valueOf String i if(numLen % 2 == 1) {
				count++;
			} numLen % 2 == 1 numLen % 2 numLen 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 = Integer.parseInt(sc.next());
		sc.close();

		int count = 0;
		for(int i = 1; i <= N; i++) {
			int numLen = String.valueOf(i).length();
			if(numLen % 2 == 1) {
				count++;
			}
		}
		System.out.println(count);
	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.next());
		sc.close();

		int count = 0;
		for(int i = 1; i <= N; i++) {
			int numLen = String.valueOf(i).length();
			if(numLen % 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 = Integer.parseInt(sc.next());
		sc.close();

		int count = 0;
		for(int i = 1; i <= N; i++) {
			int numLen = String.valueOf(i).length();
			if(numLen % 2 == 1) {
				count++;
			}
		}
		System.out.println(count);
	}
}
