What was the change in the Aggregate purchase price between 2018 and 2019?
{"Number of shares repurchased": {"April 26, 2019": 29.0, "April 27, 2018": 15.0, "April 28, 2017": 22.0}, "Average price per share": {"April 26, 2019": 72.87, "April 27, 2018": 51.57, "April 28, 2017": 32.72}, "Aggregate purchase price": {"April 26, 2019": 2111.0, "April 27, 2018": 794.0, "April 28, 2017": 705.0}, "Remaining authorization at end of period": {"April 26, 2019": 1889.0, "April 27, 2018": 4000.0, "April 28, 2017": 794.0}}
aggregate_purchase_price_april_26_2019 = df["Aggregate purchase price"]["April 26, 2019"]
aggregate_purchase_price_april_27_2018 = df["Aggregate purchase price"]["April 27, 2018"]
answer = aggregate_purchase_price_april_26_2019 - aggregate_purchase_price_april_27_2018