What is the change in the contract assets from 2018 to 2019?
{"": {"Year Ended December 31,": 2019, "": 2018}, "Contract assets": {"Year Ended December 31,": 218, "": 210}, "Contract liabilities": {"Year Ended December 31,": 346, "": 359}}
contract_assets_year_ended_december_31 = df["Contract assets"]["Year Ended December 31,"]
contract_assets_ = df["Contract assets"][""]
answer = contract_assets_year_ended_december_31 - contract_assets_