12.24. A stock price is currently $30. During each two-month period for the next four months it will increase by 8% or decrease by 10%. The risk-free interest rate is 5%. Use a twostep tree to calculate the value of a derivative that pays off [max(30 − ST , 0)]2, where ST is the stock price in four months. If the derivative is American style, should it be exercised early?