import java.util.Scanner;
import java.util.stream.IntStream;

public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int N = sc.nextInt();

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

//        IntStream.rangeClosed(1, N).filter(i -> String.valueOf(i).length() % 2 != 0).sum();
    }
} import java.util.Scanner; Scanner java.util import java.util.stream.IntStream; IntStream java.util.stream public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int N = sc.nextInt();

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

//        IntStream.rangeClosed(1, N).filter(i -> String.valueOf(i).length() % 2 != 0).sum();
    }
} Main public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int N = sc.nextInt();

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

//        IntStream.rangeClosed(1, N).filter(i -> String.valueOf(i).length() % 2 != 0).sum();
    } main {
        final Scanner sc = new Scanner(System.in);

        final int N = sc.nextInt();

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

//        IntStream.rangeClosed(1, N).filter(i -> String.valueOf(i).length() % 2 != 0).sum();
    } final Scanner sc = new Scanner(System.in); sc new Scanner(System.in) final int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int cnt = 0; cnt 0 for (int i = 1; i <= N; i++) {
            if (String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        } int i = 1; int i = 1; i 1 i <= N i N i++ i++ i {
            if (String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        } {
            if (String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        } if (String.valueOf(i).length() % 2 != 0) {
                cnt++;
            } 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 {
                cnt++;
            } cnt++ cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int N = sc.nextInt();

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

//        IntStream.rangeClosed(1, N).filter(i -> String.valueOf(i).length() % 2 != 0).sum();
    }
} public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int N = sc.nextInt();

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

//        IntStream.rangeClosed(1, N).filter(i -> String.valueOf(i).length() % 2 != 0).sum();
    }
} Main
import java.util.Scanner;
import java.util.stream.IntStream;

public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int N = sc.nextInt();

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

//        IntStream.rangeClosed(1, N).filter(i -> String.valueOf(i).length() % 2 != 0).sum();
    }
}
