What is the difference in the opening and ending balances in 2019?
{"Opening balance, January 1": {"2019": 707, "2018": 636}, "Incremental costs of obtaining a contract and contract fulfillment costs": {"2019": 602, "2018": 567}, "Amortization included in operating costs": {"2019": -523, "2018": -477}, "Impairment charges included in operating costs": {"2019": -3, "2018": -19}, "Ending balance, December 31": {"2019": 783, "2018": 707}}
ending_balance_2019 = df["Ending balance, December 31"]["2019"]
opening_balance_2019 = df["Opening balance, January 1"]["2019"]
answer = ending_balance_2019 - opening_balance_2019