What is the change in Interest income from Fiscal Year Ended December 28, 2019 to December 29, 2018?
{"": {"December 28, 2019": "", "December 29, 2018": "(Dollars in thousands)", "December 30, 2017": ""}, "Interest income": {"December 28, 2019": 2714, "December 29, 2018": 1356, "December 30, 2017": 548}, "Weighted average balance of cash and investments": {"December 28, 2019": 179526, "December 29, 2018": 138467, "December 30, 2017": 124637}, "Weighted average yield on cash and investments  ": {"December 28, 2019": 2.05, "December 29, 2018": 1.51, "December 30, 2017": 0.84}, "Interest expense": {"December 28, 2019": 1915, "December 29, 2018": 3314, "December 30, 2017": 4491}, "Average debt outstanding": {"December 28, 2019": 56776, "December 29, 2018": 90086, "December 30, 2017": 127598}, "Weighted average interest rate on debt": {"December 28, 2019": 4.09, "December 29, 2018": 3.98, "December 30, 2017": 3.07}}
interest_2019 = df["Interest income"]["December 28, 2019"]
interest_2018 = df["Interest income"]["December 29, 2018"]
answer = interest_2019 - interest_2018