import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();
    int ans = 90909;

    if(n < 10){
      ans = n;
    } else if(n < 100){
      ans = 9;
    } else if(n < 1000){
      ans = n - 90;
    } else if(n < 10000){
      ans = 909;
    } else if(n < 100000){
      ans = n -9090;
    }

    System.out.println(ans);
  }
}

/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