What was the change in expense accrued during the period between 2018 and 2019?
{"Balance at beginning of period": {"April 26, 2019": 40, "April 27, 2018": 50}, "Expense accrued during the period": {"April 26, 2019": 22, "April 27, 2018": 16}, "Warranty costs incurred": {"April 26, 2019": -22, "April 27, 2018": -26}, "Balance at end of period": {"April 26, 2019": 40, "April 27, 2018": 40}, "Total warranty liabilities": {"April 26, 2019": 40, "April 27, 2018": 40}}
expense_accrued_2019 = df["Expense accrued during the period"]["April 26, 2019"]
expense_accrued_2018 = df["Expense accrued during the period"]["April 27, 2018"]
answer = expense_accrued_2019 - expense_accrued_2018