import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int count = 0;
        for (int i = 1; i <= n; i++) {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                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 n = sc.nextInt();
        int count = 0;
        for (int i = 1; i <= n; i++) {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                count++;
            }
        }
        System.out.println(count);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int count = 0;
        for (int i = 1; i <= n; i++) {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                count++;
            }
        }
        System.out.println(count);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int count = 0;
        for (int i = 1; i <= n; i++) {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                count++;
            }
        }
        System.out.println(count);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int count = 0; count 0 for (int i = 1; i <= n; i++) {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                count++;
            }
        } int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                count++;
            }
        } {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                count++;
            }
        } int digit = 0; digit 0 int num = i; num i while (num != 0) {
                num /= 10;
                digit++;
            } num != 0 num 0 {
                num /= 10;
                digit++;
            } num /= 10 num 10 digit++ digit if (digit % 2 == 1) {
                count++;
            } digit % 2 == 1 digit % 2 digit 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 = sc.nextInt();
        int count = 0;
        for (int i = 1; i <= n; i++) {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                count++;
            }
        }
        System.out.println(count);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int count = 0;
        for (int i = 1; i <= n; i++) {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                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 n = sc.nextInt();
        int count = 0;
        for (int i = 1; i <= n; i++) {
            int digit = 0;
            int num = i;
            while (num != 0) {
                num /= 10;
                digit++;
            }
            if (digit % 2 == 1) {
                count++;
            }
        }
        System.out.println(count);
    }
}
