What is the change in Contract assets between December 31, 2018 and 2019?
{"Contract assets": {"2019": 3208206, "2018": 2759315}, "Contract liabilities": {"2019": -3887685, "2018": -2486111}, "   Net contract assets": {"2019": -679479, "2018": 273204}}
contract_assets_2019 = df["Contract assets"]["2019"]
contract_assets_2018 = df["Contract assets"]["2018"]
answer = contract_assets_2019 - contract_assets_2018