What is the change between 2018 and 2019 direct GHG emissions in tonnes?
{"": {"For the year ended 31 December": 2019.0, "": 2018.0}, "Total GHG emissions (Scopes 1 and 2) (tonnes)": {"For the year ended 31 December": 743287.01, "": 612521.16}, "Direct GHG emissions (Scope 1) (tonnes)": {"For the year ended 31 December": 316.35, "": 36.76}, "Including: Diesel (tonnes)": {"For the year ended 31 December": 316.35, "": 36.76}, "Indirect GHG emissions (Scope 2) (tonnes)": {"For the year ended 31 December": 742970.66, "": 612484.4}, "Including: Purchased electricity (tonnes)": {"For the year ended 31 December": 742970.66, "": 612484.4}, "Hazardous waste (tonnes)": {"For the year ended 31 December": 8.0, "": "\u2013"}, "Non-hazardous waste (tonnes)": {"For the year ended 31 December": 1811.27, "": 1350.76}}
direct_ghg_2019 = df["Direct GHG emissions (Scope 1) (tonnes)"]["For the year ended 31 December"]
direct_ghg_2018 = df["Direct GHG emissions (Scope 1) (tonnes)"][""]
answer = direct_ghg_2019 - direct_ghg_2018