import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		if(a==100000){
			System.out.println(90909);
	  }else if(a>=10000){
			System.out.println(909+a-9999);
		}else if(a>=1000){
			System.out.println(909);
		}else if(a>=100){
			System.out.println(9+a-99);
		}else if(a>=10){
			System.out.println(9);
		}else{
			System.out.println(a);
		}
	}
}
/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