import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int x = sc.nextInt();
		int count = 0;
		for(int i = 1;i<=x;i++) {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)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 x = sc.nextInt();
		int count = 0;
		for(int i = 1;i<=x;i++) {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)count++;
		}
		System.out.println(count);
	}

} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int x = sc.nextInt();
		int count = 0;
		for(int i = 1;i<=x;i++) {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)count++;
		}
		System.out.println(count);
	} main {
		Scanner sc = new Scanner(System.in);
		int x = sc.nextInt();
		int count = 0;
		for(int i = 1;i<=x;i++) {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)count++;
		}
		System.out.println(count);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int x = sc.nextInt(); x sc.nextInt() sc.nextInt sc int count = 0; count 0 for(int i = 1;i<=x;i++) {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)count++;
		} int i = 1; int i = 1; i 1 i<=x i x i++ i++ i {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)count++;
		} {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)count++;
		} String temp = String.valueOf(i); temp String.valueOf(i) String.valueOf String i if(temp.length()%2!=0)count++; temp.length()%2!=0 temp.length()%2 temp.length() temp.length temp 2 0 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 x = sc.nextInt();
		int count = 0;
		for(int i = 1;i<=x;i++) {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)count++;
		}
		System.out.println(count);
	}

} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int x = sc.nextInt();
		int count = 0;
		for(int i = 1;i<=x;i++) {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)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 x = sc.nextInt();
		int count = 0;
		for(int i = 1;i<=x;i++) {
			String temp = String.valueOf(i);
			if(temp.length()%2!=0)count++;
		}
		System.out.println(count);
	}

}
