import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int number, count;
    number = Integer.parseInt(sc.next());
    if(number < 9) {
      count = number;
    } else if(number < 100) {
      count = 9;
    } else if(number < 999) {
      count = 9 + (number - 99);
    } else if(number < 10000) {
      count = 909;
    } else if(number < 99999) {
      count = 9 + 900 + (number - 9999);
    } else {
      count = 90909;
    }
    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