The current price of a certain stock is $20. Suppose that over the next two years, the stock price will follow a two-step binomial tree model. Specifically, each year the stock price will either increase by 100% (i.e., u=2) or decrease by 50% (i.e., d=0.5). The stock pays no dividends, and interest rates are 25% with annual compounding. Lets consider a two-year American-style derivative security whose payoff is given by when it is exercised at any time . The price of the derivative should be $__________. (Here, represents the stock price today, the stock price in one year, and the stock price in two years.)?