import java.util.*;


public class Main {
    public static void main(String[] args){

        Scanner s = new Scanner(System.in);

      	int sc = s.nextInt();

        int judge = 0;
        for (int i = 1; i < sc+1; i++) {
            if((String.valueOf( i ).length())%2 ==1){
                judge++;
            }
        }
        System.out.println(judge);

    }
}
/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