What was the percentage change in the Balance as of 31 December from 2018 to 2019?
{"Partners and commercial managements": {"2019": 0.5, "2018": 1.2}, "Accrued operating expenses": {"2019": 14.1, "2018": 9.1}, "Accrued interest": {"2019": 4.0, "2018": 4.6}, "Wages and social expenses": {"2019": 14.3, "2018": 16.1}, "Derivative financial instruments": {"2019": 12.3, "2018": 3.4}, "Payables to joint ventures": {"2019": 0.1, "2018": 0.1}, "Other": {"2019": 2.0, "2018": 2.0}, "Balance as of 31 December": {"2019": 47.3, "2018": 36.5}}
balance_2019 = df["Balance as of 31 December"]["2019"]
balance_2018 = df["Balance as of 31 December"]["2018"]
answer = (balance_2019 - balance_2018) / balance_2018 * 100.0