What was the change in the average price per share between 2017 and 2018?
{"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}}
average_price_per_share_april_27_2018 = df["Average price per share"]["April 27, 2018"]
average_price_per_share_april_28_2017 = df["Average price per share"]["April 28, 2017"]
answer = average_price_per_share_april_27_2018 - average_price_per_share_april_28_2017