import java.util.Scanner;

class Main{
  public static void main(String[] arg){
    Scanner scan=new Scanner(System.in);
    int num=scan.nextInt();
    int Ans=0;
    if(num<10){
      Ans+=num;
    }else{
      Ans+=9;
    }
    if(num>=100 && num<1000){
      Ans+=num-99;
    }else if(num>=1000){
      Ans+=900;
    }
    if(num>=10000 && num<100000){
      Ans+=num-9999;
    }else if(num>=100000){
      Ans+=90000;
    }
    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