What is the change in balance at end of year in 2019 from 2018?
{"": {"2019": "(Dollars in millions)", "2018": "", "2017": ""}, "Balance at beginning of year": {"2019": 190, "2018": 115, "2017": 95}, "Accretion expense": {"2019": 11, "2018": 10, "2017": 6}, "Liabilities assumed in acquisition of Level 3(1)": {"2019": "\u2014", "2018": 58, "2017": 45}, "Liabilities settled": {"2019": -14, "2018": -14, "2017": -3}, "Liabilities transferred to Cyxtera": {"2019": "\u2014", "2018": "\u2014", "2017": -20}, "Change in estimate": {"2019": 10, "2018": 21, "2017": -8}, "Balance at end of year": {"2019": 197, "2018": 190, "2017": 115}}
balance_at_end_of_year_2019 = df["Balance at end of year"]["2019"]
balance_at_beginning_of_year_2019 = df["Balance at beginning of year"]["2019"]
answer = balance_at_end_of_year_2019 - balance_at_beginning_of_year_2019