What is the difference in other assets between 2018 and 2019?
{"": {"2019": "$M", "2018": "$M"}, "Property, plant and equipment": {"2019": 209, "2018": 666}, "Other assets": {"2019": 16, "2018": 155}, "Total assets classified as held for sale": {"2019": 225, "2018": 821}, "Provisions": {"2019": "\u2013", "2018": 21}, "Total liabilities directly associated with assets held for sale": {"2019": "\u2013", "2018": 21}}
other_assets_2018 = df["Other assets"]["2018"]
other_assets_2019 = df["Other assets"]["2019"]
answer = other_assets_2018 - other_assets_2019