What was the percentage change in the net interest income (expense) between 2018 and 2019?
{"Interest income": {"July 27, 2019": 1308, "July 28, 2018": 1508, "July 29, 2017": 1338, "Variance in Dollars": -200}, "Interest expense": {"July 27, 2019": -859, "July 28, 2018": -943, "July 29, 2017": -861, "Variance in Dollars": 84}, "Interest income (expense), net": {"July 27, 2019": 449, "July 28, 2018": 565, "July 29, 2017": 477, "Variance in Dollars": -116}}
net_interest_income_2019 = df["Interest income (expense), net"]["July 27, 2019"]
net_interest_income_2018 = df["Interest income (expense), net"]["July 28, 2018"]
answer = (net_interest_income_2019 - net_interest_income_2018) / net_interest_income_2018 * 100.0