import java.util.*;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int len = String.valueOf(n).length();
		int c = 0;

		for(int i = 1; i <= len; i += 2) {
			if(i < len) {
				c += (int)Math.pow(10, i - 1) * 9;
			} else {
				if(i == 1) c += n;
				else c += n - (int)Math.pow(10, i - 1) + 1;
			}
		}
		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