import java.util.*;
public class Main {

 public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
	int a=sc.nextInt();
	String nine="090909";
	int loga=(int)Math.log10(a);
	if(loga%2==1){
		 System.out.println(Integer.parseInt(nine.substring(0,(loga+1))));
	}else{
		int f=0;
		if(loga>0)f=Integer.parseInt(nine.substring(0,loga));
		System.out.println((int)(f+(a-Math.pow(10,loga)+1)));
	}
 }
}

/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