import java.util.Scanner;

public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int ans = 0;
    for(int i = 1;i <= n;i++){
      if(i < 10 || i >= 100 && i < 1000 || i >= 10000 && i < 100000) ans++;   
    }
    System.out.println(ans);
  }
}

/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