What was the change in contract assets between 2018 and 2019?
{"Prepaid expenses": {"2019": 37.1, "2018": 43.9}, "Contract assets": {"2019": 19.5, "2018": 14.8}, "Other current assets": {"2019": 8.2, "2018": 8.6}, "Prepaid expenses and other current assets": {"2019": 64.8, "2018": 67.3}}
contract_assets_2019 = df["Contract assets"]["2019"]
contract_assets_2018 = df["Contract assets"]["2018"]
answer = contract_assets_2019 - contract_assets_2018