What is the percentage 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_2019 = df["Contract assets"]["Year Ended December 31,"]
contract_assets_2018 = df["Contract assets"][""]
answer = (contract_assets_2019 - contract_assets_2018) / contract_assets_2018 * 100.0