What is the change in net income per diluted share between 2018 and 2019?
{"Net income per share: -- Basic": {"December 27, 2019 ": 0.82, "December 28, 2018 ": 0.71, "December 29, 2017": 0.55}, "Net income per share: -- Diluted": {"December 27, 2019 ": 0.81, "December 28, 2018 ": 0.7, "December 29, 2017": 0.54}, "Weighted average common shares: -- Basic": {"December 27, 2019 ": 29532342.0, "December 28, 2018 ": 28703265.0, "December 29, 2017": 26118482.0}, "Weighted average common shares: -- Diluted": {"December 27, 2019 ": 30073338.0, "December 28, 2018 ": 29678919.0, "December 29, 2017": 27424526.0}}
net_income_2019 = df["Net income per share: -- Diluted"]["December 27, 2019 "]
net_income_2018 = df["Net income per share: -- Diluted"]["December 28, 2018 "]
answer = net_income_2019 - net_income_2018