import java.util.Scanner;

public class Main {
  private static boolean isOddNumDigits (int n) {
    int count = 0;
    while (n > 0) {
      n /= 10;
      count++;
    }
    return count % 2 == 1;
  }
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = Integer.parseInt(sc.nextLine());
    sc.close();
    int count = 0;
    for (int i = 1; i <= n; i++) {
      count += isOddNumDigits(i) ? 1 : 0;
    }
    System.out.println(count);
  }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz