What was the change in the balance at the beginning of period between 2018 and 2019?
{"Balance at beginning of period": {"2019": 1868, "2018": 2366, "2017": 2226}, "Increases for tax position related to: -- Prior years": {"2019": "\u2014", "2018": 3, "2017": 465}, "Increases for tax position related to: -- Current year": {"2019": 161, "2018": 254, "2017": 285}, "Decreases for tax positions related to: -- Prior years": {"2019": -71, "2018": "\u2014", "2017": -14}, "Decreases for tax positions related to: -- Expiration of applicable statute of limitations": {"2019": -471, "2018": -755, "2017": -596}, "Decreases for tax positions related to: -- Balance at end of period": {"2019": 1487, "2018": 1868, "2017": 2366}}
balance_2019 = df["Balance at beginning of period"]["2019"]
balance_2018 = df["Balance at beginning of period"]["2018"]
answer = balance_2019 - balance_2018