
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt();
		s.close();
		int numbers = 0;
		if(n <= 9) {
			numbers = n;
		}else if(n <= 99) {
			numbers = 9;	
		}else if(n <= 999) {
			numbers = n - 90;
		}else if(n <= 9999) {
			numbers = 909;
		}else if(n <= 99999){
			numbers = n - 9000 - 90; 
		}else {
			numbers = 90909;
		}
		System.out.println(numbers);
	}

}

/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