What was the change in base tolling fee between 2018 and 2019?
{"Base tolling fee (1)": {"2019": 204501, "2018": 119677}, "Amortization of deferred commissioning period revenue (2)": {"2019": 4220, "2018": 2467}, "Amortization of Day 1 gain (3)": {"2019": 9950, "2018": 5817}, "Other": {"2019": -575, "2018": -336}, "Total": {"2019": 218096, "2018": 127625}}
base_tolling_fee_1_2019 = df["Base tolling fee (1)"]["2019"]
base_tolling_fee_1_2018 = df["Base tolling fee (1)"]["2018"]
answer = base_tolling_fee_1_2019 - base_tolling_fee_1_2018