# How to price a Perpetual American Put Option with the Binomial Tree Model?

How do we price an American Put Option with simplified assumptions of non-zero interest rate but zero volatility and zero dividend rate?

I understand the concept of Perpetual American Options and I know we need to find optimal boundary constraints. But I fail to understand how we can price the Perpetual American Put Option with an interest rate (say) r and strike price (say) K.