Piper Company had 8,500 units in ending inventory on December 31, 2018. During 2018, the company's variable production costs were $4 per unit and its fixed manufacturing overhead rate was $8 per unit. If the company's operating income for 2018 was $13,920 higher under variable costing than it was under absorption costing, how many units were in beginning inventory on January 1, 2018? (Assume the company uses normal costing)