import java.util.Scanner;

public class Main {

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

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

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        int i = 1;
        while(n >= i) {
            if (String.valueOf(i).length() % 2 == 1) {
                cnt++;
            }
            i++;
        }
        System.out.println(cnt);
    }
} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int cnt = 0;
        int i = 1;
        while(n >= i) {
            if (String.valueOf(i).length() % 2 == 1) {
                cnt++;
            }
            i++;
        }
        System.out.println(cnt);
    }
}