import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		int ans = 0;

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

		for (int i = 1; i <= N; i++) {
			String s = Integer.toString(i);
			if (s.length() % 2 != 0) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		int ans = 0;

		for (int i = 1; i <= N; i++) {
			String s = Integer.toString(i);
			if (s.length() % 2 != 0) {
				ans++;
			}
		}
		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		int ans = 0;

		for (int i = 1; i <= N; i++) {
			String s = Integer.toString(i);
			if (s.length() % 2 != 0) {
				ans++;
			}
		}
		System.out.println(ans);
	} 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 ans = 0; ans 0 for (int i = 1; i <= N; i++) {
			String s = Integer.toString(i);
			if (s.length() % 2 != 0) {
				ans++;
			}
		} int i = 1; int i = 1; i 1 i <= N i N i++ i++ i {
			String s = Integer.toString(i);
			if (s.length() % 2 != 0) {
				ans++;
			}
		} {
			String s = Integer.toString(i);
			if (s.length() % 2 != 0) {
				ans++;
			}
		} String s = Integer.toString(i); s Integer.toString(i) Integer.toString Integer i if (s.length() % 2 != 0) {
				ans++;
			} s.length() % 2 != 0 s.length() % 2 s.length() s.length s 2 0 {
				ans++;
			} ans++ ans System.out.println(ans) System.out.println System.out System System.out ans 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 ans = 0;

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

		for (int i = 1; i <= N; i++) {
			String s = Integer.toString(i);
			if (s.length() % 2 != 0) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		int ans = 0;

		for (int i = 1; i <= N; i++) {
			String s = Integer.toString(i);
			if (s.length() % 2 != 0) {
				ans++;
			}
		}
		System.out.println(ans);
	}
}
