
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

		int count = 0;

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

		int N = scan.nextInt();

		int count = 0;

		for(int i =1; i < N+1 ; i++) {
			if( String.valueOf(i).length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();
	}
} Main public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

		int count = 0;

		for(int i =1; i < N+1 ; i++) {
			if( String.valueOf(i).length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();
	} main {
		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

		int count = 0;

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

		int N = scan.nextInt();

		int count = 0;

		for(int i =1; i < N+1 ; i++) {
			if( String.valueOf(i).length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();
	}
} public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

		int count = 0;

		for(int i =1; i < N+1 ; i++) {
			if( String.valueOf(i).length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();
	}
} Main

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);

		int N = scan.nextInt();

		int count = 0;

		for(int i =1; i < N+1 ; i++) {
			if( String.valueOf(i).length()%2 == 1) {
				count++;
			}
		}
		System.out.println(count);
		scan.close();
	}
}
