What was the change in prepaid assets between 2018 and 2019?
{"Unbilled receivables": {"As of December 31, 2019": 142.3, "As of December 31, 2018": 126.1}, "Prepaid income tax": {"As of December 31, 2019": 185.8, "As of December 31, 2018": 125.1}, "Value added tax and other consumption tax receivables": {"As of December 31, 2019": 71.3, "As of December 31, 2018": 86.3}, "Prepaid assets": {"As of December 31, 2019": 56.8, "As of December 31, 2018": 40.5}, "Prepaid operating ground leases": {"As of December 31, 2019": "\u2014", "As of December 31, 2018": 165.0}, "Other miscellaneous current assets": {"As of December 31, 2019": 57.4, "As of December 31, 2018": 78.2}, "Prepaid and other current assets": {"As of December 31, 2019": 513.6, "As of December 31, 2018": 621.2}}
prepaid_assets_2019 = df["Prepaid assets"]["As of December 31, 2019"]
prepaid_assets_2018 = df["Prepaid assets"]["As of December 31, 2018"]
answer = prepaid_assets_2019 - prepaid_assets_2018