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 a = 0;
		for(int i=1; i<=N; i++) {
			int valLen = String.valueOf(i).length();
			if(valLen%2==1) {
				a++;
			}
		}
		System.out.println(a);
	}

} 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 a = 0;
		for(int i=1; i<=N; i++) {
			int valLen = String.valueOf(i).length();
			if(valLen%2==1) {
				a++;
			}
		}
		System.out.println(a);
	}

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

} public class Main {

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

} 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 a = 0;
		for(int i=1; i<=N; i++) {
			int valLen = String.valueOf(i).length();
			if(valLen%2==1) {
				a++;
			}
		}
		System.out.println(a);
	}

}