What was the percentage change in the Balance as of 31 December in 2019 from 2018?
{"Allowance for expected credit loss -- Balance as of 1 January": {"2019": 1.7, "2018": 1.3, "2017": 2.6}, "Allowance for expected credit loss -- Adjustment to prior years": {"2019": 1.5, "2018": "-", "2017": "-"}, "Allowance for expected credit loss -- Provisions for the year": {"2019": 2.4, "2018": 1.7, "2017": 0.6}, "Allowance for expected credit loss -- Provisions reversed during the year": {"2019": -1.9, "2018": -1.0, "2017": -1.9}, "Allowance for expected credit loss -- Provisions utilized during the year": {"2019": "-", "2018": -0.3, "2017": "-"}, "Allowance for expected credit loss -- Balance as of 31 December": {"2019": 3.7, "2018": 1.7, "2017": 1.3}}
balance_2019 = df["Allowance for expected credit loss -- Balance as of 31 December"]["2019"]
balance_2018 = df["Allowance for expected credit loss -- Balance as of 1 January"]["2019"]
answer = ( balance_2019 - balance_2018 ) / balance_2018 * 100.0