Competitions

# October 7 - BMTK Programming School, High League

# Forbidden Strings

A string of letters **A**, **B**, **C** is forbidden if there are three consecutive letters from which one is **A**, one is **B** and one is **C**. For example, **BAACAACCBAAA** is forbidden, while **AABBCCAABB** is not.

How many such strings of length **n** are not forbidden?

#### Input

Each line contains one number **n** (**1** ≤ **n** ≤ **30**).

#### Output

For each input value of **n** print in a separate line the number of strings of length **n** are not forbidden.

Input example #1

2 3 4

Output example #1

9 21 51