What was the change in the balance at 31st December in 2019 from 2018?
{"": {"2019": "\u00a3m", "2018": "\u00a3m"}, "Balance at 1st January": {"2019": 9.8, "2018": 9.6}, "Additional impairment": {"2019": 8.6, "2018": 2.8}, "Amounts written off as uncollectable": {"2019": -1.2, "2018": -0.7}, "Amounts recovered": {"2019": -0.6, "2018": -0.5}, "Impairment losses reversed": {"2019": -1.1, "2018": -1.4}, "Exchange differences": {"2019": -0.7, "2018": "\u2013"}, "Balance at 31st December": {"2019": 14.8, "2018": 9.8}}
balance_31_dec_2019 = df["Balance at 31st December"]["2019"]
balance_1_jan_2019 = df["Balance at 1st January"]["2019"]
answer = balance_31_dec_2019 - balance_1_jan_2019