What is the change in the gross margin between 2018 and 2019?
{"": {"2018": "", "2017": "(dollars in thousands)", "$": "", "%": ""}, "Cost of revenue": {"2018": 46810.0, "2017": 31503.0, "$": 15307.0, "%": 48.6}, "Gross margin %": {"2018": 68, "2017": 70, "$": "", "%": ""}}
gross_margin_percent_2018 = df["Gross margin %"]["2018"]
gross_margin_percent_2017 = df["Gross margin %"]["2017"]
answer = gross_margin_percent_2018 - gross_margin_percent_2017