Trent needed pinnies for the lacrosse team to wear during scrimmages. A local supplier told Trent that he could purchase pinnies for $10 each. He asked the supplier to hold five pinnies for him to purchase. However, when Trent arrived at the store a few days later, he found the price of the pinnies had increased to $12. Due to the price increase, Trent only purchased four pinnies. Trent's own-price elasticity of the pinnies, calculated using the mid-point formula, is __________.