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+1;i++){
			if(String.valueOf(i).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 scan = new Scanner(System.in);
		int n=scan.nextInt();
		int count=0;

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