import java.util.*;

class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int ct = 0;
    int digit;
    for (int i=1;i<=N;i++){
      digit = 0;
      double tmp = i;
      while (tmp>=1){
        tmp = Math.floor(tmp/10);
        digit++;
      }
      if ((digit%2) == 1){
        ct++;
      }
    }
    
    
    System.out.println(ct);
  }
}
/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