import java.util.*;
public class Main {
		
	public static void main(String[] args) {		
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		sc.close();
		
		int count = 0;
		for(int i=1;i<=N;i++) {
			String S = Integer.toString(i);
			// 111 -> "111"
			int length = S.length();
			// "111" -> 3
			if(length % 2 == 1)count++;
		}
		System.out.println(count);
	}
	
}
/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