import java.util.*;

public class Main {

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

        int cnt = 0;
        for (int i = 1; i <= n; ++i) {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        }

        System.out.println(cnt);
    }
} import java.util.*; util.* java public class Main {

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

        int cnt = 0;
        for (int i = 1; i <= n; ++i) {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        }

        System.out.println(cnt);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int cnt = 0;
        for (int i = 1; i <= n; ++i) {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        }

        System.out.println(cnt);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int cnt = 0;
        for (int i = 1; i <= n; ++i) {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        }

        System.out.println(cnt);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int cnt = 0; cnt 0 for (int i = 1; i <= n; ++i) {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        } int i = 1; int i = 1; i 1 i <= n i n ++i ++i i {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        } {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        } String s = Integer.toString(i); s Integer.toString(i) Integer.toString Integer i if (s.length() % 2 == 1) ++cnt; s.length() % 2 == 1 s.length() % 2 s.length() s.length s 2 1 ++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) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int cnt = 0;
        for (int i = 1; i <= n; ++i) {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        }

        System.out.println(cnt);
    }
} public class Main {

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

        int cnt = 0;
        for (int i = 1; i <= n; ++i) {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        }

        System.out.println(cnt);
    }
} Main
import java.util.*;

public class Main {

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

        int cnt = 0;
        for (int i = 1; i <= n; ++i) {
            String s = Integer.toString(i);
            if (s.length() % 2 == 1) ++cnt;
        }

        System.out.println(cnt);
    }
}