CF630C Lucky Numbers

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

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