PROBLEM STATEMENT
There is a number of people in a room, and each of them wears a hat which is either black or white.
Every person counts the number of other people wearing white hats. You are given a vector <int> 
count, the i-th element of which is the number counted by the i-th person. Return the total number 
of people wearing white hats, or -1 if count doesn't correspond to a valid situation.


DEFINITION
Class:WhiteHats
Method:whiteNumber
Parameters:vector <int>
Returns:int
Method signature:int whiteNumber(vector <int> count)


CONSTRAINTS
-count will contain between 2 and 50 elements, inclusive.
-Each element of count will be between 0 and 50, inclusive.


EXAMPLES

0)
{2,1,1}

Returns: 2

The first person wears a black hat and sees two people wearing white hats.  Each person wearing a 
white hat sees only one other white hat in the room.

1)
{2,2,2}

Returns: 3

Everyone wears a white hat here.

2)
{0,0}

Returns: 0

Black hats only.

3)
{1,1,1,2}

Returns: -1



4)
{10,10}

Returns: -1

Now that's interesting.  There are only two people in the room, yet each of them counted 10 others.

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