What was the change in Interest income 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}}
interest_income_July_27_2019 = df["Interest income"]["July 27, 2019"]
interest_income_July_28_2018 = df["Interest income"]["July 28, 2018"]
answer = interest_income_July_27_2019 - interest_income_July_28_2018