What is the change in beginning balance between 2018 and 2019?
{"Balance at beginning of year": {"2019": 520, "2018": 557}, "Warranty costs incurred": {"2019": -398, "2018": -40}, "Product warranty accrual": {"2019": 407, "2018": 3}, "Balance at end of year": {"2019": 529, "2018": 520}}
balance_at_beginning_of_year_2019 = df["Balance at beginning of year"]["2019"]
balance_at_beginning_of_year_2018 = df["Balance at beginning of year"]["2018"]
answer = balance_at_beginning_of_year_2019 - balance_at_beginning_of_year_2018