What was the change in Interest expense between 2017 and 2018?
{"Net sales (1)": {"2019": 1382818, "2018": 1200181, "2017": 757338}, "Operating costs and expenses: -- Cost of sales (1)": {"2019": 924276, "2018": 860744, "2017": 571944}, "Operating costs and expenses: -- Selling, general and administrative expenses": {"2019": 202642, "2018": 173620, "2017": 107658}, "Operating costs and expenses: -- Research and development (1)": {"2019": 44612, "2018": 39114, "2017": 26693}, "Operating costs and expenses: -- Restructuring charges": {"2019": 8779, "2018": 14843, "2017": 5404}, "Operating costs and expenses: -- (Gain) loss on write down and disposal of long-lived assets": {"2019": 1660, "2018": -992, "2017": 10671}, "Operating costs and expenses: -- Total operating costs and expenses": {"2019": 1181969, "2018": 1087329, "2017": 722370}, "Operating costs and expenses: -- Operating income (1)": {"2019": 200849, "2018": 112852, "2017": 34968}, "Non-operating (income) expense: -- Interest income": {"2019": -2035, "2018": -809, "2017": -24}, "Non-operating (income) expense: -- Interest expense": {"2019": 21239, "2018": 32882, "2017": 39755}, "Non-operating (income) expense: -- Acquisition (gain) loss": {"2019": "\u2014", "2018": -130880, "2017": "\u2014"}, "Non-operating (income) expense: -- Change in value of TOKIN options": {"2019": "\u2014", "2018": "\u2014", "2017": -10700}, "Non-operating (income) expense: -- Other (income) expense, net (1)": {"2019": 11214, "2018": 24592, "2017": -3871}, "Non-operating (income) expense: -- Income before income taxes and equity income (loss) from equity method investments (1)": {"2019": 170431, "2018": 187067, "2017": 9808}, "Non-operating (income) expense: -- Income tax expense (benefit) (1)": {"2019": -39460, "2018": 9132, "2017": 4294}, "Non-operating (income) expense: -- Income before equity income (loss) from equity method investments (1)": {"2019": 209891, "2018": 177935, "2017": 5514}, "Non-operating (income) expense: -- Equity income (loss) from equity method investments": {"2019": -3304, "2018": 76192, "2017": 41643}, "Non-operating (income) expense: -- Net income (1)": {"2019": 206587, "2018": 254127, "2017": 47157}}
interest_expense_2018 = df["Non-operating (income) expense: -- Interest expense"]["2018"]
interest_expense_2017 = df["Non-operating (income) expense: -- Interest expense"]["2017"]
answer = interest_expense_2018 - interest_expense_2017