What is the change in total fees between fiscal years 2018 and 2019?
{"": {"2019": "(U.S. $ in thousands)", "2018": ""}, "Audit fees (1)": {"2019": 2980, "2018": 3469}, "Audit-related fees (2)": {"2019": 648, "2018": 447}, "Tax fees (3)": {"2019": 220, "2018": 238}, "Other fees (4)": {"2019": 8, "2018": 3}, "Total": {"2019": 3856, "2018": 4157}}
total_2019 = df["Total"]["2019"]
total_2018 = df["Total"]["2018"]
answer = total_2019 - total_2018