A gambler plays a game that allows him to win a dollar when a coin flip turns up heads or to lose a dollar when tails turns up.

A gambler plays a game that allows him to win a dollar when a coin flip turns up heads or to lose a dollar when tails turns up.  Construct a box model to represent this system—one box representing the net outcome of 4 successive trials of the system.  What is the expected value of the numbers in this box?