PROBLEM STATEMENT
Fox Ciel likes shuffling cards.
She uses a deck with 2N cards, numbered 1 through 2N.
Initially, this deck is sorted: the cards are in the order 1,2,3,...,2N from top to bottom.

Ciel always uses the same procedure when shuffling.
One round of shuffling looks as follows:

She splits the deck into two piles: the top N cards will be pile A, the bottom N cards pile B.
She takes pile A and rearranges the cards it contains arbitrarily.
She takes pile B and rearranges the cards it contains arbitrarily.
She interleaves the cards from the two piles, producing a single deck again. More precisely, if 
pile A has cards A1,A2,...,AN and pile B has cards B1,B2,...,BN then the new deck will be 
A1,B1,A2,B2,...,AN,BN. (Note that the first card must be from pile A.)


For example, let N=2 and suppose that Ciel starts with the sorted deck 1,2,3,4.
After the first round of shuffling, she can produce one of the following four decks:

1,3,2,4
1,4,2,3
2,3,1,4
2,4,1,3


You are given a vector <int> permutation which contains a permutation of the 2N cards.
Ciel wants to produce this permutation of her deck.
Compute and return the smallest number of rounds of shuffling she has to perform.
(When rearranging a pile during the shuffling, she can always pick the best possible order of 
those cards.)
If Ciel cannot reach the given permutation by shuffling, return -1 instead.


DEFINITION
Class:ShufflingCardsDiv1
Method:shuffle
Parameters:vector <int>
Returns:int
Method signature:int shuffle(vector <int> permutation)


CONSTRAINTS
-permutation will contain between 4 and 2000 elements, inclusive.
-The number of elements in permutation will be even.
-The elements of permutation will form a permutation of the numbers 1 through 2N, where 2N is the 
number of elements in permutation.


EXAMPLES

0)
{1,2,3,4}

Returns: 0

There's no need to shuffle the deck at all.

1)
{1,4,3,2}

Returns: 2

One optimal solution:

In the first round, pile A will be 1,2 and pile B will be 3,4. She then merges them to produce the 
deck 1,3,2,4.
In the second round, pile A will be 1,3 and pile B will be 2,4. She rearranges pile B to 4,2 and 
then merges the two piles to produce the desired deck 1,4,3,2.


2)
{6,3,5,2,4,1}

Returns: 4



3)
{8,5,4,9,1,7,6,10,3,2}

Returns: 2



4)
{9,1,7,2,10,3,6,4,8,5}

Returns: 4



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