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