import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int count = 0;
		int ans = 0;
		int num = 0;
		for(int i = 1; i <= a; i++) {
			num = i;
			while(num != 0) {
				num = num / 10;
				count++;
			}
			if(!(count % 2 == 0)){
				ans++;
			}
			count = 0;
		}
		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