What was the change in sales and marketing expenses from 2017 to 2018?
{"": {"2019": "", "2018": "(In thousands)", "2017": ""}, "Cost of revenue": {"2019": 2843, "2018": 2435, "2017": 1406}, "Research and development": {"2019": 6532, "2018": 4283, "2017": 2968}, "Sales and marketing": {"2019": 9069, "2018": 8267, "2017": 5481}, "General and administrative": {"2019": 10693, "2018": 11476, "2017": 9114}, "Total": {"2019": 29137, "2018": 26461, "2017": 18969}}
sales_and_marketing_2018 = df["Sales and marketing"]["2018"]
sales_and_marketing_2017 = df["Sales and marketing"]["2017"]
answer = sales_and_marketing_2018 - sales_and_marketing_2017