What was the change in  Research and development in 2019 from 2018?
{"Cost of sales": {"2019": 4880, "2018": 4403, "2017": 3541}, "Research and development": {"2019": 2990, "2018": 3247, "2017": 2973}, "Selling, general and administrative": {"2019": 28596, "2018": 25088, "2017": 23911}, "Income tax benefit": {"2019": -4946, "2018": -5073, "2017": -7073}, "": {"2019": 31520, "2018": 27665, "2017": 23352}}
research_and_development_2019 = df["Research and development"]["2019"]
research_and_development_2018 = df["Research and development"]["2018"]
answer = research_and_development_2019 - research_and_development_2018