import java.util.*;
public class Main{
	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int ans = 0;
		for(int i = 1;i < N+1;i++){
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		}
		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 ans = 0;
		for(int i = 1;i < N+1;i++){
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		}
		System.out.println(ans);
	}
} Main public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int ans = 0;
		for(int i = 1;i < N+1;i++){
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		}
		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int ans = 0;
		for(int i = 1;i < N+1;i++){
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				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 int ans = 0; ans 0 for(int i = 1;i < N+1;i++){
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		} int i = 1; int i = 1; i 1 i < N+1 i N+1 N 1 i++ i++ i {
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		} {
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		} if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			} i < 10 i 10 {
				ans++;
			} ans++ ans if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			} i > 99 && i < 1000 i > 99 i 99 i < 1000 i 1000 {
				ans++;
			} ans++ ans if(i > 9999 && i < 100000){
				ans++;
			} i > 9999 && i < 100000 i > 9999 i 9999 i < 100000 i 100000 {
				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();
		int ans = 0;
		for(int i = 1;i < N+1;i++){
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		}
		System.out.println(ans);
	}
} public class Main{
	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int ans = 0;
		for(int i = 1;i < N+1;i++){
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		}
		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 ans = 0;
		for(int i = 1;i < N+1;i++){
			if(i < 10){
				ans++;
			}else if(i > 99 && i < 1000){
				ans++;
			}else if(i > 9999 && i < 100000){
				ans++;
			}
		}
		System.out.println(ans);
	}
}