What is the change in listed bonds from 2018 to 2019?
{"At 31 March": {"2019": "\u00a3m", "2018": "\u00a3m", "2017": "\u00a3m"}, "Current liabilities -- Listed bonds": {"2019": 1367, "2018": 1702, "2017": 1539}, "Current liabilities -- Finance leases": {"2019": 16, "2018": 18, "2017": 15}, "Current liabilities -- Bank loans": {"2019": "\u2013", "2018": "\u2013", "2017": 352}, "Current liabilities -- Other loans and bank overdrafts a": {"2019": 717, "2018": 561, "2017": 726}, "Current liabilities -- Total current liabilities": {"2019": 2100, "2018": 2281, "2017": 2632}, "Non-current liabilities -- Listed bonds": {"2019": 14586, "2018": 11789, "2017": 9866}, "Non-current liabilities -- Finance leases": {"2019": 190, "2018": 205, "2017": 214}, "Non-current liabilities -- Other loans": {"2019": "\u2013", "2018": "\u2013", "2017": 1}, "Non-current liabilities -- Total non-current liabilities": {"2019": 14776, "2018": 11994, "2017": 10081}, "Non-current liabilities -- Total": {"2019": 16876, "2018": 14275, "2017": 12713}}
listed_bonds_2019 = df["Current liabilities -- Listed bonds"]["2019"]
listed_bonds_2018 = df["Current liabilities -- Listed bonds"]["2018"]
answer = listed_bonds_2019 - listed_bonds_2018