import java.util.*;
public class Main{
        public static void main(String[] args){
                Scanner sc = new Scanner(System.in);
                int n = sc.nextInt();
                int yono = 0;
                for(int i = 1 ; i <= n ; i++){
                        int keta = (int)Math.log10(i);
                        keta = keta % 2;
                        if(keta == 0)yono++;
                }
                System.out.println(yono);
        }
}//0=48
 //a=97 a -> +26 = 123
//鶏になれ
/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