import java.util.Scanner;

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