import java.util.Scanner;

public class Main {

  public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in);
    int num1 = scanner.nextInt();
    int sum = 0;
    for(int i = 1; i <= num1; i++) {
    	if(String.valueOf(i).length() % 2 == 1) {
    		sum++;
    	}
    }
    System.out.println(sum);
  }
}

/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