import java.util.Scanner;


public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		String n = s.next();
		int b = Integer.parseInt(n);
		int c = 0;
		for (Integer i = 1; i <= b; i++) {
			if(i.toString().length() % 2 == 1) {
				c++;
			}
		}
		System.out.println(c);
	}

}

/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