What was the change in Total audit fees in 2019 from 2018?
{"": {"Year-ended 31 March 2019": "$M", "Year-ended 31 March 2018": "$M"}, "Audit of the Financial Statements": {"Year-ended 31 March 2019": 0.4, "Year-ended 31 March 2018": 0.4}, "Subsidiary local statutory audits": {"Year-ended 31 March 2019": 0.2, "Year-ended 31 March 2018": 0.3}, "Total audit fees": {"Year-ended 31 March 2019": 0.6, "Year-ended 31 March 2018": 0.7}, "Other assurance services": {"Year-ended 31 March 2019": 0.1, "Year-ended 31 March 2018": 0.1}, "Total non-audit fees": {"Year-ended 31 March 2019": 0.1, "Year-ended 31 March 2018": 0.1}}
total_audit_fees_2019 = df["Total audit fees"]["Year-ended 31 March 2019"]
total_audit_fees_2018 = df["Total audit fees"]["Year-ended 31 March 2018"]
answer = total_audit_fees_2019 - total_audit_fees_2018