Consider a 5-month American put option on a non-dividend paying stock when the stock price (S) is $50, the strike price (K) is $50, the risk free

Consider a 5-month American put option on a non-dividend paying stock when the stock price (S) is $50, the strike price (K) is $50, the risk free interest rate is 10% per annum, and the volatility is 40% per annum. We divide the life of the option into five intervals of length 1 month. What is p equal to?