What was the percentage change in Interest expense in 2019 from 2018?
{"": {"2019": "", "2018": "(in millions)", "2018-2019 Change": ""}, "Interest income": {"2019": -24.8, "2018": -26.7, "2018-2019 Change": 1.9}, "Interest expense": {"2019": 23.1, "2018": 31.3, "2018-2019 Change": -8.2}, "Other (income) expense, net": {"2019": 29.5, "2018": 1.4, "2018-2019 Change": 28.1}}
interest_expense_2019 = df["Interest expense"]["2019"]
interest_expense_2018 = df["Interest expense"]["2018"]
answer = ( interest_expense_2019 - interest_expense_2018 ) / interest_expense_2018 * 100.0