What is the change in contract assets in 2019?
{"": {"2019": "(Dollars in millions)", "2018": ""}, "Prepaid expenses": {"2019": 274, "2018": 307}, "Income tax receivable": {"2019": 35, "2018": 82}, "Materials, supplies and inventory": {"2019": 105, "2018": 120}, "Contract assets": {"2019": 42, "2018": 52}, "Contract acquisition costs": {"2019": 178, "2018": 167}, "Contract fulfillment costs": {"2019": 115, "2018": 82}, "Other": {"2019": 59, "2018": 108}, "Total other current assets": {"2019": 808, "2018": 918}}
contract_assets_2019 = df["Contract assets"]["2019"]
contract_assets_2018 = df["Contract assets"]["2018"]
answer = contract_assets_2019 - contract_assets_2018