
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++){
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
            
         /*   int count =0;
            while(i!=0){
                i=i/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++){
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
            
         /*   int count =0;
            while(i!=0){
                i=i/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++){
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
            
         /*   int count =0;
            while(i!=0){
                i=i/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++){
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
            
         /*   int count =0;
            while(i!=0){
                i=i/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++){
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        } int i = 1 ; int i = 1 ; i 1 i<=n i n i++ i++ i {
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        } {
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        } double j =0; j 0 j = (int)log10(i)+1 j (int)log10(i)+1 (int)log10(i) log10 i 1 if(j%2==1){
                sum++;
            } j%2==1 j%2 j 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++){
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
            
         /*   int count =0;
            while(i!=0){
                i=i/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++){
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
            
         /*   int count =0;
            while(i!=0){
                i=i/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++){
            
          
           double j =0;
           j = (int)log10(i)+1;
           
            if(j%2==1){
                sum++;
            }
        }
            
         /*   int count =0;
            while(i!=0){
                i=i/10;
                count++;
            }
            if(count%2==1){
                sum++ ;
            }
        }*/
        System.out.println(sum);
          
    }
}
