import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
public class Main
{
  
  public static void main(String[] args) throws IOException
  {
    BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    int N=Integer.parseInt(br.readLine());
    int ans=0;
    if(N<100){
      ans=Math.min(N,9);
    }else if (N<10000){
      ans=9+Math.min(N-99,900);
    }else{
      ans=9+900+Math.min(N-9999,90000);
    }
    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