import java.util.*;
 
public class Main {
 
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      
      if (n<10) System.out.println(n);
      else if (n>=10 	&& n<100) System.out.println(9);
      else if (n>=100 	&& n<1000) System.out.println(9+(n-99));
      else if (n>=1000 	&& n<10000) System.out.println(909);
      else if (n>=10000 && n<100000) System.out.println(909+(n-9999));
      else if (n==100000) System.out.println(90909);
      else System.out.println(1);
      
    }
}
/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