A10184. 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 $n$ ( $1<=n<=55$ ) — 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.