What was the change in Net income per common share-diluted in 2019 from 2018?
{"Net income for basic and diluted net income per share": {"2019": 467468.0, "2018": 451779.0, "2017": 257692.0}, "Weighted average common shares-basic": {"2019": 170425.0, "2018": 187672.0, "2017": 198069.0}, "Effect of dilutive potential common shares: -- Incremental shares from assumed conversion of convertible notes (1)": {"2019": 4909.0, "2018": 2749.0, "2017": 1298.0}, "Effect of dilutive potential common shares: -- Convertible note hedge warrant shares (2)": {"2019": 2698.0, "2018": 485.0, "2017": 112.0}, "Effect of dilutive potential common shares: -- Restricted stock units": {"2019": 1236.0, "2018": 1385.0, "2017": 1800.0}, "Effect of dilutive potential common shares: -- Stock options": {"2019": 178.0, "2018": 278.0, "2017": 335.0}, "Effect of dilutive potential common shares: -- Employee stock purchase rights": {"2019": 13.0, "2018": 36.0, "2017": 27.0}, "Effect of dilutive potential common shares: -- Dilutive potential common shares": {"2019": 9034.0, "2018": 4933.0, "2017": 3572.0}, "Effect of dilutive potential common shares: -- Weighted average common shares-diluted": {"2019": 179459.0, "2018": 192605.0, "2017": 201641.0}, "Effect of dilutive potential common shares: -- Net income per common share-basic": {"2019": 2.74, "2018": 2.41, "2017": 1.3}, "Effect of dilutive potential common shares: -- Net income per common share-diluted": {"2019": 2.6, "2018": 2.35, "2017": 1.28}}
net_income_2019 = df["Effect of dilutive potential common shares: -- Net income per common share-diluted"]["2019"]
net_income_2018 = df["Effect of dilutive potential common shares: -- Net income per common share-diluted"]["2018"]
answer = net_income_2019 - net_income_2018