What was the change in contract assets between 2018 and 2019?
{"Accounts receivable": {"December 31, 2019": 90531, "December 31, 2018": 99385}, "Contract assets (1)": {"December 31, 2019": 2812, "December 31, 2018": 3766}, "Unearned revenue": {"December 31, 2019": 11963, "December 31, 2018": 17940}, "Non-current unearned revenue": {"December 31, 2019": 6012, "December 31, 2018": 5296}}
contract_assets_2019 = df["Contract assets (1)"]["December 31, 2019"]
contract_assets_2018 = df["Contract assets (1)"]["December 31, 2018"]
answer = contract_assets_2019 - contract_assets_2018