What is the change in Aggregate purchase price from year ended December 31, 2018 to 2019?
{"Number of repurchased shares (1)": {"2019": 7.8, "2018": 16.8, "2017": 1.6}, "Aggregate purchase price": {"2019": 138.9, "2018": 315.0, "2017": 25.0}, "Fees, commissions and other expenses": {"2019": 0.1, "2018": 0.3, "2017": "\u2014"}, "Total cash used for share repurchases": {"2019": 139.0, "2018": 315.3, "2017": 25.0}, "Weighted-average purchase price per share": {"2019": 17.89, "2018": 18.78, "2017": 15.35}, "Available for future purchases at period end": {"2019": 1361.1, "2018": 1500.0, "2017": 603.2}}
aggregate_purchase_price_2019 = df["Aggregate purchase price"]["2019"]
aggregate_purchase_price_2018 = df["Aggregate purchase price"]["2018"]
answer = aggregate_purchase_price_2019 - aggregate_purchase_price_2018