What is the change in cost of sales in 2019 from 2018?
{"Cost of sales": {"2019": 1766, "2018": 1096, "2017": 338}, "Selling, general and administrative": {"2019": 13722, "2018": 6419, "2017": 4674}, "": {"2019": 15488, "2018": 7515, "2017": 5012}}
cost_of_sales_2019 = df["Cost of sales"]["2019"]
cost_of_sales_2018 = df["Cost of sales"]["2018"]
answer = cost_of_sales_2019 - cost_of_sales_2018