What was the percentage change in Debt principal from 2018 to 2019?
{"": {"December 31, 2019": "(in thousands)", "December 31, 2018": ""}, "Debt principal": {"December 31, 2019": 460000, "December 31, 2018": 460000}, "Unamortized discount": {"December 31, 2019": 65313, "December 31, 2018": 80019}, "Net carrying amount of convertible debt": {"December 31, 2019": 394687, "December 31, 2018": 379981}}
debt_principal_2019 = df["Debt principal"]["December 31, 2019"]
debt_principal_2018 = df["Debt principal"]["December 31, 2018"]
answer = (debt_principal_2019 - debt_principal_2018) / debt_principal_2018 * 100.0