
import static java.lang.Math.log10;
import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(); 
        
       int sum = 0 ;
        for(int i = 1 ; i<=n ; i++){
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 }
        System.out.println(sum);
          
    }
} import static java.lang.Math.log10; import static java.lang.Math.log10; import static java.lang.Math.log10; 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 sum = 0 ;
        for(int i = 1 ; i<=n ; i++){
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 }
        System.out.println(sum);
          
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(); 
        
       int sum = 0 ;
        for(int i = 1 ; i<=n ; i++){
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 }
        System.out.println(sum);
          
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(); 
        
       int sum = 0 ;
        for(int i = 1 ; i<=n ; i++){
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 }
        System.out.println(sum);
          
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int sum = 0 ; sum 0 for(int i = 1 ; i<=n ; i++){
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 } int i = 1 ; int i = 1 ; i 1 i<=n i n i++ i++ i {
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 } {
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 } int temp = i ; temp i int count =0; count 0 while(temp!=0){
                temp=temp/10;
                count++;
            } temp!=0 temp 0 {
                temp=temp/10;
                count++;
            } temp=temp/10 temp temp/10 temp 10 count++ count if(count%2==1){
                sum++ ;
            } count%2==1 count%2 count 2 1 {
                sum++ ;
            } sum++ sum System.out.println(sum) System.out.println System.out System System.out sum String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(); 
        
       int sum = 0 ;
        for(int i = 1 ; i<=n ; i++){
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 }
        System.out.println(sum);
          
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(); 
        
       int sum = 0 ;
        for(int i = 1 ; i<=n ; i++){
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 }
        System.out.println(sum);
          
    }
} Main

import static java.lang.Math.log10;
import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(); 
        
       int sum = 0 ;
        for(int i = 1 ; i<=n ; i++){
            
          int temp = i ;
     /*      double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
       */     
           int count =0;
            while(temp!=0){
                temp=temp/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
                 }
        System.out.println(sum);
          
    }
}
