What is the change in the Business combination reserve between 2018 and 2019?
{"Reserves -- Share-based payment reserve": {"2019 $000": 3960, "2018 $000": 3198}, "Reserves -- Business combination reserve": {"2019 $000": 5571, "2018 $000": 5571}, "Reserves -- Foreign currency translation reserve": {"2019 $000": -12, "2018 $000": -24}, "Reserves -- ": {"2019 $000": 9519, "2018 $000": 8745}}
reserve_2019 = df["Reserves -- Business combination reserve"]["2019 $000"]
reserve_2018 = df["Reserves -- Business combination reserve"]["2018 $000"]
answer = reserve_2019 - reserve_2018