What was the increase / (decrease) in the balance at beginning of year from 2018 to 2019?
{"Balance at beginning of year": {"2019": 4421, "2018": 6667, "2017": 2813}, "Provision charged to expense": {"2019": 9347, "2018": 8793, "2017": 7072}, "Write-offs, less recoveries": {"2019": -9219, "2018": -11039, "2017": -6516}, "Acquired allowance for doubtful accounts": {"2019": "\u2014", "2018": "\u2014", "2017": 3298}, "Balance at end of year": {"2019": 4549, "2018": 4421, "2017": 6667}}
balance_2019 = df["Balance at beginning of year"]["2019"]
balance_2018 = df["Balance at beginning of year"]["2018"]
answer = balance_2019 - balance_2018