What is the percentage change in the total stock-based compensation expense from 2018 to 2019?
{"U.S. $ in thousands -- Research and development": {"2019": 123, "2 0 1 8": 175}, "U.S. $ in thousands -- General and administrative": {"2019": 666, "2 0 1 8": 844}, "U.S. $ in thousands -- Total stock-based compensation expense": {"2019": 789, "2 0 1 8": 1019}}
total_stockbased_compensation_expense_2019 = df["U.S. $ in thousands -- Total stock-based compensation expense"]["2019"]
total_stockbased_compensation_expense_2018 = df["U.S. $ in thousands -- Total stock-based compensation expense"]["2 0 1 8"]
answer = (total_stockbased_compensation_expense_2019 - total_stockbased_compensation_expense_2018) / total_stockbased_compensation_expense_2018 * 100.0