import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        for (int i=1;i<n+1;i++){
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        }
        System.out.println(count);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        for (int i=1;i<n+1;i++){
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        }
        System.out.println(count);
    }
} Main public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        for (int i=1;i<n+1;i++){
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        }
        System.out.println(count);
    } main {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        for (int i=1;i<n+1;i++){
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        }
        System.out.println(count);
    } Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int n = scanner.nextInt(); n scanner.nextInt() scanner.nextInt scanner int count = 0; count 0 for (int i=1;i<n+1;i++){
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        } int i=1; int i=1; i 1 i<n+1 i n+1 n 1 i++ i++ i {
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        } {
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        } if (Integer.toString(i).length()%2==1){
                count++;
            } Integer.toString(i).length()%2==1 Integer.toString(i).length()%2 Integer.toString(i).length() Integer.toString(i).length Integer.toString(i) Integer.toString Integer i 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 scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        for (int i=1;i<n+1;i++){
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        }
        System.out.println(count);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        for (int i=1;i<n+1;i++){
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        }
        System.out.println(count);
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        for (int i=1;i<n+1;i++){
            if (Integer.toString(i).length()%2==1){
                count++;
            }
        }
        System.out.println(count);
    }
}