What is the change in Cost of revenue from December 31, 2018 to 2019?
{"Cost of revenue": {"2019": 10.6, "2018": 7.0, "2017": 6.0}, "Research and development": {"2019": 17.0, "2018": 14.3, "2017": 12.5}, "Selling and marketing": {"2019": 14.8, "2018": 14.1, "2017": 11.7}, "General and administrative": {"2019": 37.0, "2018": 42.9, "2017": 39.6}, "Share-based compensation expense": {"2019": 79.4, "2018": 78.3, "2017": 69.8}, "Related income tax benefits at federal rate (1)": {"2019": -16.7, "2018": -16.4, "2017": -24.4}, "Share-based compensation expense, net of taxes": {"2019": 62.7, "2018": 61.9, "2017": 45.4}}
cost_2019 = df["Cost of revenue"]["2019"]
cost_2018 = df["Cost of revenue"]["2018"]
answer = cost_2019 - cost_2018