import java.util.Scanner;

public class Main {

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