What is the increase in the Biogas from 2018 to 2019?
{"Biogas (MWh)": {"2019": 17045, "2018": 15958}, "Natural gas (MWh)": {"2019": 238081, "2018": 278849}, "PG (MWh)": {"2019": 866, "2018": 1513}, "Fuel oils (MWh)": {"2019": 580, "2018": 648}, "Coal (MWh)": {"2019": 0, "2018": 0}, "Electricity (MWh)": {"2019": 195796, "2018": 196965}, "Heat and steam (MWh)": {"2019": 212482, "2018": 272985}, "Total energy (MWh)(a)": {"2019": 408280, "2018": 469950}, "Total Scope 1 emissions (tonnes CO2e)": {"2019": 48178, "2018": 56533}, "Total Scope 2 emissions (tonnes COe)(b)2": {"2019": 702, "2018": 3067}}
biogas_MWh_2019 = df["Biogas (MWh)"]["2019"]
biogas_MWh_2018 = df["Biogas (MWh)"]["2018"]
answer = biogas_MWh_2019 - biogas_MWh_2018