Walmart has purchased 500 swimsuits before the start of the summer season at a cost of $20 each (for a total cost of $10,000). The season lasts four months, and Walmart has forecast demand in each of the four months to be d1 = 500 - p1, d2 = 500 - 1.2p2, d3 = 500 - 1.5p3 and d4 = 500 - 1.8 p4.

Formulate the problem to maximize Walmart revenue.