What was the percentage change in the net interest expense between 2018 and 2019?
{"Net income (U.S. GAAP) (1)": {"2019": 206587, "2018": 254127, "2017": 47157}, "Non-GAAP adjustments: -- Income tax expense (benefit)": {"2019": -39460, "2018": 9132, "2017": 4294}, "Non-GAAP adjustments: -- Interest expense, net": {"2019": 19204, "2018": 32073, "2017": 39731}, "Non-GAAP adjustments: -- Depreciation and amortization": {"2019": 52628, "2018": 50661, "2017": 38151}, "Non-GAAP adjustments: -- EBITDA (non-GAAP) (1)": {"2019": 238959, "2018": 345993, "2017": 129333}, "Excluding the following items: -- Equity (income) loss from equity method investments": {"2019": 3304, "2018": -76192, "2017": -41643}, "Excluding the following items: -- Acquisition (gain) loss": {"2019": "\u2014", "2018": -130880, "2017": "\u2014"}, "Excluding the following items: -- Change in value of TOKIN options": {"2019": "\u2014", "2018": "\u2014", "2017": -10700}, "Excluding the following items: -- (Gain) loss on write down and disposal of long-lived assets": {"2019": 1660, "2018": -992, "2017": 10671}, "Excluding the following items: -- ERP integration costs/IT transition costs": {"2019": 8813, "2018": 80, "2017": 7045}, "Excluding the following items: -- Stock-based compensation": {"2019": 12866, "2018": 7657, "2017": 4720}, "Excluding the following items: -- Restructuring charges (2)": {"2019": 8779, "2018": 14843, "2017": 5404}, "Excluding the following items: -- R&D grant reimbursements and grant income": {"2019": -4559, "2018": "\u2014", "2017": "\u2014"}, "Excluding the following items: -- Legal expenses/fines related to antitrust class actions": {"2019": 11896, "2018": 16636, "2017": 2640}, "Excluding the following items: -- Net foreign exchange (gain) loss": {"2019": -7230, "2018": 13145, "2017": -3758}, "Excluding the following items: -- TOKIN investment-related expenses": {"2019": "\u2014", "2018": "\u2014", "2017": 1101}, "Excluding the following items: -- Plant start-up costs (2)": {"2019": -927, "2018": 929, "2017": 427}, "Excluding the following items: -- Loss on early extinguishment of debt": {"2019": 15946, "2018": 486, "2017": "\u2014"}, "Excluding the following items: -- Adjusted EBITDA (non-GAAP) (1)": {"2019": 289507, "2018": 191705, "2017": 105240}}
net_interest_expense_2019 = df["Non-GAAP adjustments: -- Interest expense, net"]["2019"]
net_interest_expense_2018 = df["Non-GAAP adjustments: -- Interest expense, net"]["2018"]
answer = ( net_interest_expense_2019 - net_interest_expense_2018 ) / net_interest_expense_2018 * 100.0