What is the change in balance at start of year from 2018 to 2019?
{"Balance January 1": {"2018": 6562, "2019": 7955}, "Charged to cost of sales": {"2018": 18408, "2019": 26301}, "Deductions": {"2018": -8985, "2019": -12232}, "Releases of expired warranty": {"2018": -8214, "2019": -5684}, "Foreign currency translation effect": {"2018": 184, "2019": 84}, "Balance December 31": {"2018": 7955, "2019": 16424}}
balance_january_1_2018 = df["Balance January 1"]["2018"]
balance_january_1_2019 = df["Balance January 1"]["2019"]
answer = balance_january_1_2019 - balance_january_1_2018