What was the change in the amount of research and development between 2018 and 2019?
{"Stock-based compensation expense data: -- Sales and marketing": {"2019": 2075, "2018": 1196, "2017": 561, "2016": 536, "2015": 372}, "Stock-based compensation expense data: -- General and administrative": {"2019": 6474, "2018": 4901, "2017": 2638, "2016": 1430, "2015": 2486}, "Stock-based compensation expense data: -- Research and development": {"2019": 12054, "2018": 7332, "2017": 4214, "2016": 2035, "2015": 1266}, "Stock-based compensation expense data: -- Total stock-based compensation expense": {"2019": 20603, "2018": 13429, "2017": 7413, "2016": 4001, "2015": 4124}}
research_and_development_2019 = df["Stock-based compensation expense data: -- Research and development"]["2019"]
research_and_development_2018 = df["Stock-based compensation expense data: -- Research and development"]["2018"]
answer = research_and_development_2019 - research_and_development_2018