What was the change in trade receivables?
{"": {"2019": "$ million", "2018": "$ million"}, "Neither impaired nor past due": {"2019": 115.3, "2018": 99.8}, "Past due but not impaired: --  Less than 30 days overdue": {"2019": 8.6, "2018": 16.8}, "Past due but not impaired: --  30 to 60 days": {"2019": 3.6, "2018": 3.2}, "Past due but not impaired: --  Over 60 days": {"2019": 1.2, "2018": 3.6}, "Past due but not impaired: -- Trade receivables": {"2019": 128.7, "2018": 123.4}}
trade_receivables_2019 = df["Past due but not impaired: -- Trade receivables"]["2019"]
trade_receivables_2018 = df["Past due but not impaired: -- Trade receivables"]["2018"]
answer = trade_receivables_2019 - trade_receivables_2018