B Inc. buys 2000 units of flappers every year for $ 1800 per flapper. The holding cost per item per year is $ 1200 and the cost per order is $400. The supplier offers a 2% discount B Inc. orders in a minimum lot size of 50 units (they can order more than 50, but not less than 50 in each order to get the price discount). If B Inc. wants to get a price discount, how many units should they order at one time?