What is the change in revenue between 2018 and 2019?
{"Consolidated Statements of Income Data: -- Revenue": {"2019": 70697.0, "2018": 55838.0, "2017": 40653.0, "2016": 27638.0, "2015": 17928.0}, "Consolidated Statements of Income Data: -- Total costs and expenses(1)": {"2019": 46711.0, "2018": 30925.0, "2017": 20450.0, "2016": 15211.0, "2015": 11703.0}, "Consolidated Statements of Income Data: -- Income from operations": {"2019": 23986.0, "2018": 24913.0, "2017": 20203.0, "2016": 12427.0, "2015": 6225.0}, "Consolidated Statements of Income Data: -- Income before provision for income taxes": {"2019": 24812.0, "2018": 25361.0, "2017": 20594.0, "2016": 12518.0, "2015": 6194.0}, "Consolidated Statements of Income Data: -- Net income": {"2019": 18485.0, "2018": 22112.0, "2017": 15934.0, "2016": 10217.0, "2015": 3688.0}, "Consolidated Statements of Income Data: -- Net income attributable to Class A and Class B common stockholders": {"2019": 18485.0, "2018": 22111.0, "2017": 15920.0, "2016": 10188.0, "2015": 3669.0}, "Earnings per share attributable to Class A and Class B common stockholders: -- Basic": {"2019": 6.48, "2018": 7.65, "2017": 5.49, "2016": 3.56, "2015": 1.31}, "Earnings per share attributable to Class A and Class B common stockholders: -- Diluted": {"2019": 6.43, "2018": 7.57, "2017": 5.39, "2016": 3.49, "2015": 1.29}}
revenue_2019 = df["Consolidated Statements of Income Data: -- Revenue"]["2019"]
revenue_2018 = df["Consolidated Statements of Income Data: -- Revenue"]["2018"]
answer = revenue_2019 - revenue_2018