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