import java.util.Scanner;

public 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++) {
            if(String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
} 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 cnt = 0;
        for(int i=1; i<=n; i++) {
            if(String.valueOf(i).length() % 2 != 0) {
                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++) {
            if(String.valueOf(i).length() % 2 != 0) {
                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++) {
            if(String.valueOf(i).length() % 2 != 0) {
                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++) {
            if(String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        } int i=1; int i=1; i 1 i<=n i n i++ i++ i {
            if(String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        } {
            if(String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        } if(String.valueOf(i).length() % 2 != 0) {
                cnt++;
            } String.valueOf(i).length() % 2 != 0 String.valueOf(i).length() % 2 String.valueOf(i).length() String.valueOf(i).length String.valueOf(i) String.valueOf String i 2 0 {
                cnt++;
            } cnt++ cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args public 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++) {
            if(String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
} public 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++) {
            if(String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
} 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 cnt = 0;
        for(int i=1; i<=n; i++) {
            if(String.valueOf(i).length() % 2 != 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
}
