import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String N = in.next();
        int res = 0;

        for (int i = 1; i <= Integer.parseInt(N); i++)
            if (String.valueOf(i).length() % 2 == 1) res++;
        System.out.println(res);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String N = in.next();
        int res = 0;

        for (int i = 1; i <= Integer.parseInt(N); i++)
            if (String.valueOf(i).length() % 2 == 1) res++;
        System.out.println(res);
    }
} Main public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String N = in.next();
        int res = 0;

        for (int i = 1; i <= Integer.parseInt(N); i++)
            if (String.valueOf(i).length() % 2 == 1) res++;
        System.out.println(res);
    } main {
        Scanner in = new Scanner(System.in);
        String N = in.next();
        int res = 0;

        for (int i = 1; i <= Integer.parseInt(N); i++)
            if (String.valueOf(i).length() % 2 == 1) res++;
        System.out.println(res);
    } Scanner in = new Scanner(System.in); in new Scanner(System.in) String N = in.next(); N in.next() in.next in int res = 0; res 0 for (int i = 1; i <= Integer.parseInt(N); i++)
            if (String.valueOf(i).length() % 2 == 1) res++; int i = 1; int i = 1; i 1 i <= Integer.parseInt(N) i Integer.parseInt(N) Integer.parseInt Integer N i++ i++ i if (String.valueOf(i).length() % 2 == 1) res++; if (String.valueOf(i).length() % 2 == 1) res++; String.valueOf(i).length() % 2 == 1 String.valueOf(i).length() % 2 String.valueOf(i).length() String.valueOf(i).length String.valueOf(i) String.valueOf String i 2 1 res++ res System.out.println(res) System.out.println System.out System System.out res String[] args args public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String N = in.next();
        int res = 0;

        for (int i = 1; i <= Integer.parseInt(N); i++)
            if (String.valueOf(i).length() % 2 == 1) res++;
        System.out.println(res);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String N = in.next();
        int res = 0;

        for (int i = 1; i <= Integer.parseInt(N); i++)
            if (String.valueOf(i).length() % 2 == 1) res++;
        System.out.println(res);
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String N = in.next();
        int res = 0;

        for (int i = 1; i <= Integer.parseInt(N); i++)
            if (String.valueOf(i).length() % 2 == 1) res++;
        System.out.println(res);
    }
}
