import java.util.Scanner;
import java.util.ArrayList;
import java.util.Collections;

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 import java.util.ArrayList; ArrayList java.util import java.util.Collections; Collections 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;
import java.util.ArrayList;
import java.util.Collections;

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);
  }
}
