CF535B.Tavas and SaDDas

Once again Tavas started eating coffee mix without water! Keione told him that it smells awful, but he didn't stop doing that. That's why Keione told his smart friend, SaDDas to punish him! SaDDas took Tavas' headphones and told him: "If you solve the following problem, I'll return it to you."

The problem is:

You are given a lucky number n. Lucky numbers are the positive integers whose decimal representations contain only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.

If we sort all lucky numbers in increasing order, what's the 1-based index of n?

Tavas is not as smart as SaDDas, so he asked you to do him a favor and solve this problem so he can have his headphones back.

塔瓦斯又开始不加水直接吃咖啡粉了！凯奥恩告诉他这味道太难闻，但他还是不停止。因此，凯奥恩叫来了他聪明的朋友萨达斯来惩罚他！萨达斯拿走了塔瓦斯的耳机，并对他说：“如果你能解决下面这个问题，我就把耳机还给你。”

问题是：

给定一个幸运数 $n$。幸运数是指其十进制表示中仅包含幸运数字 4 和 7 的正整数。例如，47、744、4 是幸运数，而 5、17、467 则不是。

如果我们把所有幸运数按升序排列，那么 $n$ 在该序列中的从 1 开始计数的位置（即 1-based index） 是多少？

塔瓦斯可没萨达斯那么聪明，所以他请你帮个忙，解决这个问题，好让他能把耳机拿回来。

The first and only line of input contains a lucky number n (1 ≤ n ≤ 109).

输入仅有一行，包含一个幸运数字 $n$（$1 \leq n \leq 10^9$）。

Print the index of n among all lucky numbers.

输出 n 在所有幸运数中的索引。