What was the change in the Amortization of debt issuance costs between 2017 and 2018?
{"Contractual interest expense": {"2019": 19471, "2018": 30323, "2017": 38825}, "Capitalized interest": {"2019": -232, "2018": -141, "2017": -154}, "Amortization of debt issuance costs": {"2019": 334, "2018": 511, "2017": 1390}, "Amortization of debt (premium) discount": {"2019": 1481, "2018": 1843, "2017": -788}, "Imputed interest on acquisition related obligations": {"2019": 57, "2018": 113, "2017": 159}, "Interest expense on capital leases": {"2019": 128, "2018": 233, "2017": 323}, "Total interest expense": {"2019": 21239, "2018": 32882, "2017": 39755}}
amortization_2018 = df["Amortization of debt issuance costs"]["2018"]
amortization_2017 = df["Amortization of debt issuance costs"]["2017"]
answer = amortization_2018 - amortization_2017