Lucky Numbers

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。

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.

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

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

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