What is the change in the gross margin between 2017 and 2018?
{"": {"2018": "", "2017": "(dollars in thousands)", "$": "", "%": ""}, "Sales and marketing": {"2018": 69608.0, "2017": 46998.0, "$": 22610.0, "%": 48.1}, "% of revenue": {"2018": 47, "2017": 45, "$": "", "%": ""}}
percent_of_revenue_2018 = df["% of revenue"]["2018"]
percent_of_revenue_2017 = df["% of revenue"]["2017"]
answer = percent_of_revenue_2018 - percent_of_revenue_2017