import java.util.Scanner;

public class Main {

    public static void main(String args[]) {

        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int result = 0;
        for (int i = n; i >= 1; i--) {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        }

        System.out.println(result);
    }
} 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 result = 0;
        for (int i = n; i >= 1; i--) {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        }

        System.out.println(result);
    }
} Main public static void main(String args[]) {

        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int result = 0;
        for (int i = n; i >= 1; i--) {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        }

        System.out.println(result);
    } main {

        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int result = 0;
        for (int i = n; i >= 1; i--) {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        }

        System.out.println(result);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int result = 0; result 0 for (int i = n; i >= 1; i--) {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        } int i = n; int i = n; i n i >= 1 i 1 i-- i-- i {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        } {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        } if (Integer.toString(i).length() % 2 == 1) {
                result++;
            } 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 {
                result++;
            } result++ result System.out.println(result) System.out.println System.out System System.out result String args[] args public class Main {

    public static void main(String args[]) {

        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int result = 0;
        for (int i = n; i >= 1; i--) {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        }

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

    public static void main(String args[]) {

        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        int result = 0;
        for (int i = n; i >= 1; i--) {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        }

        System.out.println(result);
    }
} 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 result = 0;
        for (int i = n; i >= 1; i--) {
            if (Integer.toString(i).length() % 2 == 1) {
                result++;
            }
        }

        System.out.println(result);
    }
}
