import java.util.*;
class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int cnt = 0;
        for(int i = 1; i<=n; i++) {
            int d = (int)Math.log10(i) + 1;
            if(d%2==1) cnt++;
        }
        System.out.println(cnt);
    }
} import java.util.*; util.* java class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int cnt = 0;
        for(int i = 1; i<=n; i++) {
            int d = (int)Math.log10(i) + 1;
            if(d%2==1) cnt++;
        }
        System.out.println(cnt);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int cnt = 0;
        for(int i = 1; i<=n; i++) {
            int d = (int)Math.log10(i) + 1;
            if(d%2==1) cnt++;
        }
        System.out.println(cnt);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int cnt = 0;
        for(int i = 1; i<=n; i++) {
            int d = (int)Math.log10(i) + 1;
            if(d%2==1) cnt++;
        }
        System.out.println(cnt);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int cnt = 0; cnt 0 for(int i = 1; i<=n; i++) {
            int d = (int)Math.log10(i) + 1;
            if(d%2==1) cnt++;
        } int i = 1; int i = 1; i 1 i<=n i n i++ i++ i {
            int d = (int)Math.log10(i) + 1;
            if(d%2==1) cnt++;
        } {
            int d = (int)Math.log10(i) + 1;
            if(d%2==1) cnt++;
        } int d = (int)Math.log10(i) + 1; d (int)Math.log10(i) + 1 (int)Math.log10(i) Math.log10 Math i 1 if(d%2==1) cnt++; d%2==1 d%2 d 2 1 cnt++ cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args
import java.util.*;
class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int cnt = 0;
        for(int i = 1; i<=n; i++) {
            int d = (int)Math.log10(i) + 1;
            if(d%2==1) cnt++;
        }
        System.out.println(cnt);
    }
}