import java.util.Scanner;

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

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

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

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

} Main
import java.util.Scanner;

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

}