import java.util.Scanner;

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

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