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