import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count =0;
    for (int i=1; i<=N; i++) {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    }
    System.out.println(count);
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count =0;
    for (int i=1; i<=N; i++) {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    }
    System.out.println(count);
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count =0;
    for (int i=1; i<=N; i++) {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    }
    System.out.println(count);
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count =0;
    for (int i=1; i<=N; i++) {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    }
    System.out.println(count);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int count =0; count 0 for (int i=1; i<=N; i++) {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    } int i=1; int i=1; i 1 i<=N i N i++ i++ i {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    } {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    } if (i <= 9) count++; i <= 9 i 9 count++ count if (i >= 100 && i<= 999) count++; i >= 100 && i<= 999 i >= 100 i 100 i<= 999 i 999 count++ count if (i >= 10000 && i<= 99999) count++; i >= 10000 && i<= 99999 i >= 10000 i 10000 i<= 99999 i 99999 count++ count System.out.println(count) System.out.println System.out System System.out count String[] args args public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count =0;
    for (int i=1; i<=N; i++) {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    }
    System.out.println(count);
  }
} public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count =0;
    for (int i=1; i<=N; i++) {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    }
    System.out.println(count);
  }
} Main
import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int count =0;
    for (int i=1; i<=N; i++) {
      if (i <= 9) count++;
      if (i >= 100 && i<= 999) count++;
      if (i >= 10000 && i<= 99999) count++;
    }
    System.out.println(count);
  }
}