What was the change in the product cost of sales between 2017 and 2018?
{"Cost of salesproduct": {"July 27, 2019": 90, "July 28, 2018": 94, "July 29, 2017": 85}, "Cost of salesservice": {"July 27, 2019": 130, "July 28, 2018": 133, "July 29, 2017": 134}, "Share-based compensation expense in cost of sales .": {"July 27, 2019": 220, "July 28, 2018": 227, "July 29, 2017": 219}, "Research and development .": {"July 27, 2019": 540, "July 28, 2018": 538, "July 29, 2017": 529}, "Sales and marketing": {"July 27, 2019": 519, "July 28, 2018": 555, "July 29, 2017": 542}, "General and administrative": {"July 27, 2019": 250, "July 28, 2018": 246, "July 29, 2017": 236}, "Restructuring and other charges": {"July 27, 2019": 62, "July 28, 2018": 33, "July 29, 2017": 3}, "Share-based compensation expense in operating expenses": {"July 27, 2019": 1371, "July 28, 2018": 1372, "July 29, 2017": 1310}, "Total share-based compensation expense": {"July 27, 2019": 1591, "July 28, 2018": 1599, "July 29, 2017": 1529}, "Income tax benefit for share-based compensation .": {"July 27, 2019": 542, "July 28, 2018": 558, "July 29, 2017": 451}}
cost_of_sales_product_2018 = df["Cost of salesproduct"]["July 28, 2018"]
cost_of_sales_product_2017 = df["Cost of salesproduct"]["July 29, 2017"]
answer = cost_of_sales_product_2018 - cost_of_sales_product_2017