What was the change in the Effect of dilutive potential common shares between 2017 and 2018?
{"Net income .": {"July 27, 2019": 11621.0, "July 28, 2018": 110.0, "July 29, 2017": 9609.0}, "Weighted-average sharesbasic": {"July 27, 2019": 4419.0, "July 28, 2018": 4837.0, "July 29, 2017": 5010.0}, "Effect of dilutive potential common shares": {"July 27, 2019": 34.0, "July 28, 2018": 44.0, "July 29, 2017": 39.0}, "Weighted-average sharesdiluted .": {"July 27, 2019": 4453.0, "July 28, 2018": 4881.0, "July 29, 2017": 5049.0}, "Net income per sharebasic .": {"July 27, 2019": 2.63, "July 28, 2018": 0.02, "July 29, 2017": 1.92}, "Net income per sharediluted": {"July 27, 2019": 2.61, "July 28, 2018": 0.02, "July 29, 2017": 1.9}, "Antidilutive employee share-based awards, excluded": {"July 27, 2019": 55.0, "July 28, 2018": 61.0, "July 29, 2017": 136.0}}
effect_2018 = df["Effect of dilutive potential common shares"]["July 28, 2018"]
effect_2017 = df["Effect of dilutive potential common shares"]["July 29, 2017"]
answer = effect_2018 - effect_2017