What is the change in the Retrospective regulatory matters from 2018 to 2019?
{"10. Specific items continued -- ": {"2019": 2019, "2018": 2018, "2017": 2017}, "10. Specific items continued -- Year ended 31 March": {"2019": "\u00a3m", "2018": "\u00a3m", "2017": "\u00a3m"}, "Revenue -- Italian business investigation": {"2019": "\u2013", "2018": "\u2013", "2017": 22}, "Revenue -- Retrospective regulatory matters": {"2019": 31, "2018": 23, "2017": -2}, "Revenue -- ": {"2019": 31, "2018": 23, "2017": 20}, "Operating costs -- EE acquisition warranty claims": {"2019": "\u2013", "2018": 225, "2017": "\u2013"}, "Operating costs -- Restructuring charges": {"2019": 386, "2018": 241, "2017": "\u2013"}, "Operating costs -- EE integration costs": {"2019": "\u2013", "2018": 46, "2017": 215}, "Operating costs -- Property rationalisation costs": {"2019": 36, "2018": 28, "2017": "\u2013"}, "Operating costs -- Pension equalisation costs": {"2019": 26, "2018": "\u2013", "2017": "\u2013"}, "Operating costs -- Retrospective regulatory matters": {"2019": -4, "2018": 26, "2017": 481}, "Operating costs -- Italian business investigation": {"2019": -55, "2018": 22, "2017": 238}, "Operating costs -- Out of period irrecoverable VAT": {"2019": "\u2013", "2018": "\u2013", "2017": 30}, "Operating costs -- Profit (loss) on disposal of businesses": {"2019": 5, "2018": -1, "2017": -16}, "Operating costs -- ": {"2019": 394, "2018": 587, "2017": 948}, "Operating costs -- Operating loss": {"2019": 425, "2018": 610, "2017": 968}, "Net finance expense -- Interest expense on retirement benefit obligation": {"2019": 139, "2018": 218, "2017": 209}, "Net finance expense -- Interest on out of period irrecoverable VAT": {"2019": "\u2013", "2018": "\u2013", "2017": 1}, "Net finance expense -- ": {"2019": 139, "2018": 218, "2017": 210}, "Net finance expense -- Net specific items charge before tax": {"2019": 564, "2018": 828, "2017": 1178}, "Taxation -- Tax credit on specific items above": {"2019": -112, "2018": -87, "2017": -154}, "Taxation -- Tax credit on re-measurement of deferred tax": {"2019": "\u2013", "2018": "\u2013", "2017": -63}, "Taxation -- ": {"2019": -112, "2018": -87, "2017": -217}, "Taxation -- Net specific items charge after tax": {"2019": 452, "2018": 741, "2017": 961}}
revenue_retrospective_regulatory_matters_2019 = df["Revenue -- Retrospective regulatory matters"]["2019"]
revenue_retrospective_regulatory_matters_2018 = df["Revenue -- Retrospective regulatory matters"]["2018"]
answer = revenue_retrospective_regulatory_matters_2019 - revenue_retrospective_regulatory_matters_2018