import java.util.*;

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

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