What is the change in Principal from December 31, 2019 to December 31, 2018?
{"Liability component: -- Principal": {"2019": 92000, "2018": 115000}, "Liability component: -- Less: debt discount, net of amortization": {"2019": -12776, "2018": -20903}, "Liability component: -- Net carrying amount": {"2019": 79224, "2018": 94097}, "Liability component: -- Equity component (1)": {"2019": -14555, "2018": 22094}}
liability_component__principal_2019 = df["Liability component: -- Principal"]["2019"]
liability_component__principal_2018 = df["Liability component: -- Principal"]["2018"]
answer = liability_component__principal_2019 - liability_component__principal_2018