Bakery Problem

For today, Chef is out of town, and his evil son has taken over his bakery. Normally, Chef sells his cakes for $100$ rupees each. However, his son can be bribed with $K$ rupees, to sell each cake for $60$ rupees instead. Note that you have to pay this bribe only one time, not for each cake. You want to buy $N$ cakes from the bakery. You can choose whether to bribe Chef's son or not. What is the