import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
	
	if(a < 10)
		System.out.println(a);
	else if(a < 100)
		System.out.println(9);
	else if(a < 1000)
		System.out.println(a - 90);
	else if(a < 10000)
		System.out.println(909);
	else if(a < 100000)
		System.out.println(a - 9090);
	else
		System.out.println(90909);
	
  }
	
}
/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