What was the change in the Employee stock purchase plan between 2018 and 2019?
{"Stock options and assumed options": {"2019": 3783, "2018": 3511, "2017": 3913}, "Restricted stock units": {"2019": 16627, "2018": 9770, "2017": 3366}, "Restricted stock awards": {"2019": "\u2014", "2018": 1, "2017": 19}, "Employee stock purchase plan": {"2019": 193, "2018": 147, "2017": 115}, "Total stock-based compensation expense": {"2019": 20603, "2018": 13429, "2017": 7413}, "Tax benefit from stock-based awards": {"2019": 5154, "2018": 7581, "2017": 12719}}
employee_stock_purchase_plan_2019 = df["Employee stock purchase plan"]["2019"]
employee_stock_purchase_plan_2018 = df["Employee stock purchase plan"]["2018"]
answer = employee_stock_purchase_plan_2019 - employee_stock_purchase_plan_2018