import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = 0;
        for(int i = 1; i <= n; i++){
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        }
        System.out.println(ans);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = 0;
        for(int i = 1; i <= n; i++){
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        }
        System.out.println(ans);
    }
} Main public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = 0;
        for(int i = 1; i <= n; i++){
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        }
        System.out.println(ans);
    } main {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = 0;
        for(int i = 1; i <= n; i++){
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        }
        System.out.println(ans);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int ans = 0; ans 0 for(int i = 1; i <= n; i++){
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        } int i = 1; int i = 1; i 1 i <= n i n i++ i++ i {
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        } {
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        } String s = String.valueOf(i); s String.valueOf(i) String.valueOf String i ans += s.length()%2 != 0 ? 1 : 0 ans s.length()%2 != 0 ? 1 : 0 s.length()%2 != 0 s.length()%2 s.length() s.length s 2 0 1 0 System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = 0;
        for(int i = 1; i <= n; i++){
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        }
        System.out.println(ans);
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = 0;
        for(int i = 1; i <= n; i++){
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        }
        System.out.println(ans);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int ans = 0;
        for(int i = 1; i <= n; i++){
            String s = String.valueOf(i);
            ans += s.length()%2 != 0 ? 1 : 0;
        }
        System.out.println(ans);
    }
}
