import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();

        if (str.length() <= 1) {
            System.out.print(Integer.parseInt(str));
            return;
        }

        if (str.length() <= 2) {
            System.out.print(9);
            return;
        }

        if (str.length() <= 3) {
            System.out.print(9 + Integer.parseInt(str) - 99);
            return;
        }

        if (str.length() <= 4) {
            System.out.print(909);
            return;
        }

        if (str.length() <= 5) {
            System.out.print(909 + Integer.parseInt(str) - 9999);
            return;
        }

        if (str.length() == 6) {
            System.out.print(90909);
            return;
        }
    }

}
/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