import java.util.*;

public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();

    if(N<=9){
    System.out.println(N);
    }else if(N<=99){
    System.out.println("9");
    }else if(N<=999){
    System.out.println(N-99+9);
    }else if(N<=9999){
    System.out.println("909");
    }else if(N<=99999){
    System.out.println(N-9999+909);
    }else{
    System.out.println("90909");    
    }
}
}
/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