import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int val1 = sc.nextInt();

        int count = 0;
        for (int i = 1; i <= val1; i++) {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        }
        System.out.println(count);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int val1 = sc.nextInt();

        int count = 0;
        for (int i = 1; i <= val1; i++) {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        }
        System.out.println(count);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int val1 = sc.nextInt();

        int count = 0;
        for (int i = 1; i <= val1; i++) {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        }
        System.out.println(count);
    } main {
        Scanner sc = new Scanner(System.in);

        int val1 = sc.nextInt();

        int count = 0;
        for (int i = 1; i <= val1; i++) {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        }
        System.out.println(count);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int val1 = sc.nextInt(); val1 sc.nextInt() sc.nextInt sc int count = 0; count 0 for (int i = 1; i <= val1; i++) {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        } int i = 1; int i = 1; i 1 i <= val1 i val1 i++ i++ i {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        } {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        } String temp = "" + i; temp "" + i "" i if ((temp.length() % 2) != 0) {
                count++;
            } (temp.length() % 2) != 0 (temp.length() % 2) temp.length() temp.length temp 2 0 {
                count++;
            } count++ count System.out.println(count) System.out.println System.out System System.out count String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int val1 = sc.nextInt();

        int count = 0;
        for (int i = 1; i <= val1; i++) {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        }
        System.out.println(count);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int val1 = sc.nextInt();

        int count = 0;
        for (int i = 1; i <= val1; i++) {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        }
        System.out.println(count);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int val1 = sc.nextInt();

        int count = 0;
        for (int i = 1; i <= val1; i++) {
            String temp = "" + i;
            if ((temp.length() % 2) != 0) {
                count++;
            }
        }
        System.out.println(count);
    }
}