What was the change in the balance at the end of period between 2018 and 2019?
{"Balance at beginning of period": {"2019": 8623, "2018": 9724, "2017": 8548}, "Plus: Amounts charged to cost and expenses": {"2019": 4569, "2018": 7392, "2017": 6951}, "Less: Deductions": {"2019": -4798, "2018": -8493, "2017": -5775}, "Balance at end of period": {"2019": 8394, "2018": 8623, "2017": 9724}}
balance_end_2019 = df["Balance at end of period"]["2019"]
balance_start_2019 = df["Balance at beginning of period"]["2019"]
answer = balance_end_2019 - balance_start_2019