import java.util.Scanner;

public class Main {

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

} 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();
		scan.close();
		int cnt = 0;
		for(int i = 1; i <=n; i++) {
			int digit = String.valueOf(i).length();
			if(digit % 2 == 1) {
				cnt++;
			}
		}
		System.out.println(cnt);
	}

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

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

} public class Main {

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

} Main
import java.util.Scanner;

public class Main {

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

}