import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        for (int i = 1; i <= n; i++) {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        }
        System.out.println(cnt);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        for (int i = 1; i <= n; i++) {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        }
        System.out.println(cnt);
    }
} Main public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        for (int i = 1; i <= n; i++) {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        }
        System.out.println(cnt);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        for (int i = 1; i <= n; i++) {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        }
        System.out.println(cnt);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = Integer.parseInt(sc.next()); n Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int cnt = 0; cnt 0 for (int i = 1; i <= n; i++) {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        } int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        } {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        } String tmp = Integer.toString(i); tmp Integer.toString(i) Integer.toString Integer i if (tmp.length() % 2 != 0) cnt++; tmp.length() % 2 != 0 tmp.length() % 2 tmp.length() tmp.length tmp 2 0 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) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        for (int i = 1; i <= n; i++) {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        }
        System.out.println(cnt);
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        for (int i = 1; i <= n; i++) {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        }
        System.out.println(cnt);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        for (int i = 1; i <= n; i++) {
            String tmp = Integer.toString(i);
            if (tmp.length() % 2 != 0) cnt++;
        }
        System.out.println(cnt);
    }
}
