What was the change in operating expenses between 2017 and 2018?
{"Revenues": {"2019 (1)": 59.5, "2018": 57.6, "2017": 56.8}, "Operating expenses": {"2019 (1)": 12.5, "2018": 12.1, "2017": 12.3}, "Guarantee fee": {"2019 (1)": "\u2014", "2018": "\u2014", "2017": 1.2}}
operating_expenses_2018 = df["Operating expenses"]["2018"]
operating_expenses_2017 = df["Operating expenses"]["2017"]
answer = operating_expenses_2018 - operating_expenses_2017