What was the change in deferred revenue in 2019 from 2018?
{"Deferred revenue": {"2019": 3012, "2018": 3655}, "Customer arrangements with termination rights": {"2019": 15024, "2018": 16404}, "Estimated refund liabilities": {"2019": 8585, "2018": 10117}, "": {"2019": 26621, "2018": 30176}}
deferred_revenue_2019 = df["Deferred revenue"]["2019"]
deferred_revenue_2018 = df["Deferred revenue"]["2018"]
answer = deferred_revenue_2019 - deferred_revenue_2018