import java.util.*;
 
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        int digit = String.valueOf(n).length();

        int sum = 0;
        for (int i = 1; i <= n; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                sum++;
            }
        }

        System.out.println(sum);
	}
} 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 digit = String.valueOf(n).length();

        int sum = 0;
        for (int i = 1; i <= n; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                sum++;
            }
        }

        System.out.println(sum);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        int digit = String.valueOf(n).length();

        int sum = 0;
        for (int i = 1; i <= n; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                sum++;
            }
        }

        System.out.println(sum);
	} main {
		Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        int digit = String.valueOf(n).length();

        int sum = 0;
        for (int i = 1; i <= n; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                sum++;
            }
        }

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

        int sum = 0;
        for (int i = 1; i <= n; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                sum++;
            }
        }

        System.out.println(sum);
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        int digit = String.valueOf(n).length();

        int sum = 0;
        for (int i = 1; i <= n; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                sum++;
            }
        }

        System.out.println(sum);
	}
} Main
import java.util.*;
 
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        int digit = String.valueOf(n).length();

        int sum = 0;
        for (int i = 1; i <= n; i++) {
            if (String.valueOf(i).length() % 2 == 1) {
                sum++;
            }
        }

        System.out.println(sum);
	}
}
