What is the change in the Derivative financial assets from 2018 to 2019?
{"At 31 March Notes": {"Notes": "", "2019": "\u00a3m", "2018": "\u00a3m", "2017": "\u00a3m"}, "Derivative financial assets": {"Notes": "", "2019": 1592, "2018": 1509, "2017": 2246}, "Investments": {"Notes": 23, "2019": 3268, "2018": 3075, "2017": 1564}, "Trade and other receivables a": {"Notes": 17, "2019": 1766, "2018": 2518, "2017": 2729}, "Contract assets": {"Notes": 6, "2019": 1602, "2018": "\u2013", "2017": "\u2013"}, "Cash and cash equivalents": {"Notes": 24, "2019": 1666, "2018": 528, "2017": 528}, "": {"Notes": "", "2019": 9894, "2018": 7630, "2017": 7067}}
derivative_financial_assets_2019 = df["Derivative financial assets"]["2019"]
derivative_financial_assets_2018 = df["Derivative financial assets"]["2018"]
answer = derivative_financial_assets_2019 - derivative_financial_assets_2018