What is the difference between 2018 and 2019 Total 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": 113501.5, "": 102831.74}, "Direct GHG emissions (Scope 1) (tonnes)": {"For the year ended 31 December": 3785.86, "": 2554.31}, "Including: Gasoline (tonnes)": {"For the year ended 31 December": 197.25, "": 191.0}, "Diesel (tonnes)": {"For the year ended 31 December": 10.87, "": 11.07}, "Natural gas (tonnes)": {"For the year ended 31 December": 3577.74, "": 2352.24}, "Indirect GHG emissions (Scope 2) (tonnes)": {"For the year ended 31 December": 109715.64, "": 100277.43}, "Including: Purchased electricity (tonnes)": {"For the year ended 31 December": 109715.64, "": 100277.43}, "Total GHG emissions per employee (tonnes per employee)": {"For the year ended 31 December": 1.9, "": 2.01}, "Total GHG emissions per floor area (tonnes per square metre)": {"For the year ended 31 December": 0.07, "": 0.09}, "Hazardous waste (tonnes)": {"For the year ended 31 December": 2.4, "": 2.51}, "Hazardous waste per employee (tonnes per employee)": {"For the year ended 31 December": 4e-05, "": 5e-05}, "Non-hazardous waste (tonnes)": {"For the year ended 31 December": 5227.11, "": 4566.52}, "Non-hazardous waste per employee (tonnes per employee)": {"For the year ended 31 December": 0.09, "": 0.09}}
total_ghg_2018 = df["Total GHG emissions (Scopes 1 and 2) (tonnes)"][""]
total_ghg_2019 = df["Total GHG emissions (Scopes 1 and 2) (tonnes)"]["For the year ended 31 December"]
answer = total_ghg_2019 - total_ghg_2018