You are informed to create a wedding cake for a couple. The couple has already picked out a design that they like. The cake is composed of three tiers. Each tier is a square prism. The bottom tier has a length of 50 cm. The second tier has a length of 35 cm, and the top tier has a length of 20 cm. Each tier has a height of 15 cm. The surface of the cake should be completely covered with frosting. How many cans of frosting will you need to buy, if each can cover 250 square cm?