描述

The numbers of all offices in the new building of the Tax Office of IT City will have lucky numbers.

A lucky number is a number that consists of digits 7 and 8 only. Find the maximum number of offices in the new building of the Tax Office given that a door-plate can hold a number not longer than $n$ digits.

中文题意

Petya喜欢幸运数字。我们称一个数字为幸运数字，当且仅当它的所有数位都是 $7$ 或 $8$ 。

现在Petya想知道：有多少个位数不超过 $n$ 的正整数是幸运数字？

数据范围：1≤n≤55

输入

The only line of input contains one integer $n$ ($1 \leq n \leq 55$) — the maximum length of a number that a door-plate can hold.

输出

Output one integer — the maximum number of offices that can have unique lucky numbers not longer than $n$ digits.

样例

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