What would be the change in balance at the beginning of the period from 2017 to 2018?
{"Balance at the beginning of the period": {"December 29, 2019": 172266, "December 30, 2018": 181303, "December 31, 2017": 161209}, "Accruals for warranties issued during the period": {"December 29, 2019": 27717, "December 30, 2018": 31628, "December 31, 2017": 29689}, "Settlements and adjustments during the period": {"December 29, 2019": -61538, "December 30, 2018": -40665, "December 31, 2017": -9595}, "Balance at the end of the period": {"December 29, 2019": 138445, "December 30, 2018": 172266, "December 31, 2017": 181303}}
balance_2018 = df["Balance at the beginning of the period"]["December 30, 2018"]
balance_2017 = df["Balance at the beginning of the period"]["December 31, 2017"]
answer = balance_2018 - balance_2017