import java.util.*;

class Main {
  public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      int N = sc.nextInt();
      int Ans = 0;
      if(N<10) {
    	  Ans = N;
      }
      else if(N<100) {
    	  Ans = 9;
      }
      else if(N<1000) {
    	  Ans = N -100 +10;
      }
      else if(N<10000) {
    	  Ans = 900 + 9;
      }
      else if(N<100000) {
    	  Ans = N-10000 + 910;
      }else {
    	  Ans = 90909;
      }
      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