What was the change in sales and marketing between 2017 and 2018?
{"Stock-based compensation expense data: -- Sales and marketing": {"2019": 2075, "2018": 1196, "2017": 561}, "Stock-based compensation expense data: -- General and administrative": {"2019": 6474, "2018": 4901, "2017": 2638}, "Stock-based compensation expense data: -- Research and development": {"2019": 12054, "2018": 7332, "2017": 4214}, "Stock-based compensation expense data: -- Total stock-based compensation expense": {"2019": 20603, "2018": 13429, "2017": 7413}}
sales_2018 = df["Stock-based compensation expense data: -- Sales and marketing"]["2018"]
sales_2017 = df["Stock-based compensation expense data: -- Sales and marketing"]["2017"]
answer = sales_2018 - sales_2017