import java.util.Scanner;

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

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