import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		 Scanner sc = new Scanner(System.in);
		 int N = sc.nextInt();
		 int count = 0;
		 for(int i=1; i<=N; i++) {
			 if((String.valueOf(i).length()) %2 == 1) {
				 count++;
			 }
		 }
		 System.out.println(count);
	}
}
/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