What was the change in the restricted stock units between 2018 and 2019?
{"Restricted stock units": {"2019": 53766, "2018": 84082, "2017": 42122}, "Employee stock purchase plan (1)": {"2019": 7580, "2018": 6891, "2017": 6334}, "Other": {"2019": "\u2014", "2018": 7538, "2017": 88}, "Total": {"2019": 61346, "2018": 98511, "2017": 48544}}
restricted_stock_units_2019 = df["Restricted stock units"]["2019"]
restricted_stock_units_2018 = df["Restricted stock units"]["2018"]
answer = restricted_stock_units_2019 - restricted_stock_units_2018