The price of a non-dividend-paying stock is $$\$ 19$$ and the price of a 3-month European call option on the stock with a strike price of $$\$ 20$$ is $$\$ 1$$. The risk-free rate is $4 \%$ per annum. What is the price of a 3-month European put option with a strike price of $$\$ 20$$ ?