import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int N = s.nextInt();
        int ans = N;
        int che = 0;
        for (int i = 1; i < (N + 1); i++) {
            che = String.valueOf(i).length();
            if (che % 2 == 0) {
                ans--;
            }
        }
            System.out.print(ans);
    }
}

/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