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; i++) {
            if ((String.valueOf(i).length() % 2 ) != 0) {
                count++;
            }
        }
        System.out.print(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; i++) {
            if ((String.valueOf(i).length() % 2 ) != 0) {
                count++;
            }
        }
        System.out.print(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; i++) {
            if ((String.valueOf(i).length() % 2 ) != 0) {
                count++;
            }
        }
        System.out.print(count) ;



    } main {

        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();

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



    }


}

