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