import java.util.*;
public class Main{
  
  public static void main(String[] args){
    
    Scanner in = new Scanner(System.in);
    int N = in.nextInt();
    int count=0;
    for(int i=1;i<=N;i++){
      String x = Integer.toString(i);
      int length = x.length();
      if(length%2!=0){
        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