Lucky Numbers

题面翻译

给定数字位数，即这个数的位数要小于或者等于它，且这个数字只能由7和8组成，问有多少种情况。

题目描述

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

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.

输入格式

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

输出格式

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

样例 #1

样例输入 #1

2

样例输出 #1

6