import java.util.*;
public class Main {
		
	public static void main(String[] args) {		
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 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();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 2 == 1)count++;
		}
		System.out.println(count);
	}
	
} Main public static void main(String[] args) {		
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 2 == 1)count++;
		}
		System.out.println(count);
	} main {		
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 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 sc.close() sc.close sc int count = 0; count 0 for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 2 == 1)count++;
		} int i=1; int i=1; i 1 i<=N i N i++ i++ i {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 2 == 1)count++;
		} {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 2 == 1)count++;
		} String S = Integer.toString(i); S Integer.toString(i) Integer.toString Integer i int length = S.length(); length S.length() S.length S if(length % 2 == 1)count++; length % 2 == 1 length % 2 length 2 1 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();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(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 = sc.nextInt();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 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();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 2 == 1)count++;
		}
		System.out.println(count);
	}
	
}