import java.util.*;

public class Main {

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    int count = 0;
    for(int i = 1;i<=n;i++){
      String s = String.valueOf(i);
      if(s.length() % 2 != 0){
        count++;
      }
    }
    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