import java.util.*;
public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        int all = 0;
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        for (int i = 1; i <= n; i++) {
            String str = i + "";
            if (str.length() % 2 == 1) {
                all++;
            }
        }
        System.out.println(all + "");
    }
}
/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