What is the change in the earned and unbilled receivables?
{"": {"2019": "(Dollars in millions)", "2018": ""}, "Trade and purchased receivables": {"2019": 1971, "2018": 2094}, "Earned and unbilled receivables": {"2019": 374, "2018": 425}, "Other": {"2019": 20, "2018": 21}, "Total accounts receivable": {"2019": 2365, "2018": 2540}, "Less: allowance for doubtful accounts": {"2019": -106, "2018": -142}, "Accounts receivable, less allowance": {"2019": 2259, "2018": 2398}}
earned_and_unbilled_receivables_2019 = df["Earned and unbilled receivables"]["2019"]
earned_and_unbilled_receivables_2018 = df["Earned and unbilled receivables"]["2018"]
answer = earned_and_unbilled_receivables_2019 - earned_and_unbilled_receivables_2018