import java.util.Scanner;

 class Main {
	public static void main(String[]args) {
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int count = 0 ;
		for(int i = 1 ; i <= n ; i++) {
			String s = Integer.toString(i);
			if(s.length() % 2 != 0) {
				count++;
			}else {
				i = (i*10)-1 ;
			}
		}
		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