import java.awt.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
import java.util.List;

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

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

} Main public static void main(String args[]) throws IOException {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int cnt=0;
        for (int i=1;i<=n;i++){
            if (String.valueOf(i).length()%2==1){
                cnt++;
            }
        }
        System.out.println(cnt);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int cnt=0;
        for (int i=1;i<=n;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 int cnt=0; cnt 0 for (int i=1;i<=n;i++){
            if (String.valueOf(i).length()%2==1){
                cnt++;
            }
        } int i=1; int i=1; i 1 i<=n i n 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[]) throws IOException {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int cnt=0;
        for (int i=1;i<=n;i++){
            if (String.valueOf(i).length()%2==1){
                cnt++;
            }
        }
        System.out.println(cnt);
    }

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

} Main
import java.awt.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
import java.util.List;

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

}

