What was the change in audit fees from 2018 to 2019?
{"": {"2018": "(Expressed in millions of U.S. Dollars)", "2019": ""}, "Audit fees": {"2018": 1.8, "2019": 1.7}, "Total fees": {"2018": 1.8, "2019": 1.7}}
audit_fees_2019 = df["Audit fees"]["2019"]
audit_fees_2018 = df["Audit fees"]["2018"]
answer = audit_fees_2019 - audit_fees_2018