What is the difference in research and development costs between 2018 and 2019?
{"Cost of revenue": {"2019": 6403, "2018": 7312, "2017": 5312}, "Research and development": {"2019": 44855, "2018": 57188, "2017": 54123}, "Sales and marketing": {"2019": 11585, "2018": 14726, "2017": 14959}, "General and administrative": {"2019": 14896, "2018": 17783, "2017": 17187}, "Total stock-based compensation expense": {"2019": 77739, "2018": 97009, "2017": 91581}}
rd_2018 = df["Research and development"]["2018"]
rd_2019 = df["Research and development"]["2019"]
answer = rd_2018 - rd_2019