What would be the change in Research and development expense between 2017 and 2018?
{"Cost of product revenues": {"April 26, 2019": 4, "April 27, 2018": 3, "April 28, 2017": 4}, "Cost of hardware maintenance and other services revenues": {"April 26, 2019": 10, "April 27, 2018": 10, "April 28, 2017": 13}, "Sales and marketing": {"April 26, 2019": 67, "April 27, 2018": 68, "April 28, 2017": 84}, "Research and development": {"April 26, 2019": 48, "April 27, 2018": 49, "April 28, 2017": 59}, "General and administrative": {"April 26, 2019": 29, "April 27, 2018": 31, "April 28, 2017": 35}, "Total stock-based compensation expense": {"April 26, 2019": 158, "April 27, 2018": 161, "April 28, 2017": 195}, "Income tax benefit for stock-based compensation": {"April 26, 2019": 15, "April 27, 2018": 29, "April 28, 2017": 41}}
rd_expense_2018 = df["Research and development"]["April 27, 2018"]
rd_expense_2017 = df["Research and development"]["April 28, 2017"]
answer = rd_expense_2018 - rd_expense_2017