import java.util.Scanner;

public class Main {

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

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