What was the change in Aggregate purchase price between 2018 and 2019?
{"Aggregate purchase price (1)": {"January 31, 2020": 1334.0, "February 1, 2019": 42.0, "February 2, 2018": 1449.0}, "Class A common stock repurchased": {"January 31, 2020": 7664.0, "February 1, 2019": 286.0, "February 2, 2018": 13977.0}, "Weighted-average price per share": {"January 31, 2020": 174.02, "February 1, 2019": 148.07, "February 2, 2018": 103.66}}
aggregate_purchase_price_1_february_1_2019 = df["Aggregate purchase price (1)"]["February 1, 2019"]
aggregate_purchase_price_1_february_2_2018 = df["Aggregate purchase price (1)"]["February 2, 2018"]
answer = aggregate_purchase_price_1_february_1_2019 - aggregate_purchase_price_1_february_2_2018