What was the change in the balance at the beginning of the period and end of the period in 2019?
{"Balance at beginning of period": {"2019": 185, "2018": 205}, "Plus: Amounts charged to cost and expense": {"2019": 6014, "2018": 7261}, "Less: Amounts paid": {"2019": -4631, "2018": -7281}, "Balance at end of period": {"2019": 1568, "2018": 185}}
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