What is the total research and development expenses in 2018 and 2019?
{"": {"2019": "", "2018": "(dollars in thousands)", "$": "", "%": ""}, "Research and development": {"2019": 17845, "2018": 22450, "$": -4605, "%": -21}, "Percent of revenues, net": {"2019": 36, "2018": 38, "$": "", "%": ""}}
rd_expenses_2018 = df["Research and development"]["2018"]
rd_expenses_2019 = df["Research and development"]["2019"]
answer = rd_expenses_2018 + rd_expenses_2019