import java.util.*;

public class Main {
  public static void main(String[] args ) throws Exception {
    Scanner sc = new Scanner(System.in);
    int Number = sc.nextInt();
    int count=0;
    int valLen =0;
    for(int i=1; i<=Number;i++){
      valLen= String.valueOf( i ).length();
    	if(valLen%2==1){
        count++;
        }
    }System.out.println(count);
  }
}


/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