What was the change in the Principal from 2018 to 2019?
{"Principal": {"December 31, 2019": 212000, "December 31, 2018": 262000}, "Less: -- Unamortized debt discount": {"December 31, 2019": -1328, "December 31, 2018": -1630}, "Less: -- Unamortized debt issuance costs": {"December 31, 2019": -3763, "December 31, 2018": -4613}, "Less: -- Net carrying amount of long-term debt": {"December 31, 2019": 206909, "December 31, 2018": 255757}, "Less: -- Less: current portion of long-term debt": {"December 31, 2019": "\u2014", "December 31, 2018": "\u2014"}, "Less: -- Long-term debt, non-current portion": {"December 31, 2019": 206909, "December 31, 2018": 255757}}
principal_december_31_2019 = df["Principal"]["December 31, 2019"]
principal_december_31_2018 = df["Principal"]["December 31, 2018"]
answer = principal_december_31_2019 - principal_december_31_2018