import java.util.Scanner;

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

	int N = sc.nextInt();
	int n = (String.valueOf(N)).length();

	int count = 0;
	int x = 10;
	int i;
	for (i = 1; i < n; i++, x *= 10) {
	    if ((i % 2) == 1) 
		count += x - (x / 10);
	}
	if ((i % 2) == 1)
	    count += N - (x / 10) + 1;

	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