import java.util.*;
public class Main {
public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
  	int N = sc.nextInt();
 	int cnt = 0;	

  	for (int i=N;i>0;i--){
      if(((0<i)&&(i<10))
         ||((99<i)&&(i<1000))
         ||((9999<i)&&(i<100000))){
         cnt++;
      }
    }
 
  	System.out.println(cnt);
  
	}
}
/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