
import java.util.*;

public class Main {

    public static void main(String[] args) {

        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        StringBuilder ans;
//        long cnt = 0;
        int cnt = 0;
        for (int i = 1; i <= n; i++) {

            ans = new StringBuilder(""+i);
            if (ans.length() % 2 != 0) {
                cnt++;
            }
        }

        System.out.println(cnt);

    }

}

/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