import java.util.*;

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

        if(N<10){
        System.out.print(N);
        }else if(N>=10 && N<100){
        System.out.print("9");
        }else if(100<=N && N<1000){
        System.out.print(9+(N-99));
        }else if(1000<=N && N<10000){
        System.out.print("909");        
        }else if(10000<=N && N<100000){
        System.out.print(909+(N-9999));
        }else{
         System.out.print("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