PROBLEM STATEMENT
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.

Your task is to calculate how many such strings of length n are not forbidden.


DEFINITION
Class:ForbiddenStrings
Method:countNotForbidden
Parameters:int
Returns:long long
Method signature:long long countNotForbidden(int n)


CONSTRAINTS
-n will be between 1 and 30, inclusive.


EXAMPLES

0)
2

Returns: 9

All 9 strings of length 2 are not forbidden.


1)
3

Returns: 21

There are 27 strings of length 3. Of these, 6 contain one occurrence of each letter. Those 6 
strings are forbidden, so you should return 21.


2)
4

Returns: 51



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 unauthorized use or reproduction of this information without the prior written consent of 
 TopCoder, Inc. is strictly prohibited. (c)2010, TopCoder, Inc. All rights reserved.
 
  This problem statement is the exclusive and proprietary property of TopCoder, Inc. Any unauthorized use or reproduction of this information without the prior written consent of TopCoder, Inc. is strictly prohibited. (c)2010, TopCoder, Inc. All rights reserved.