What is the change in sales and marketing between 2018 and 2019?
{"": {"2019": "", "2018": "(dollars in thousands)", "2017": ""}, "Sales and marketing": {"2019": 227733, "2018": 224635, "2017": 240271}, "Percent of revenue": {"2019": 39.5, "2018": 41.8, "2017": 49.9}}
sales_and_marketing_2019 = df["Sales and marketing"]["2019"]
sales_and_marketing_2018 = df["Sales and marketing"]["2018"]
answer = ( sales_and_marketing_2019 - sales_and_marketing_2018 )