What is the change in trade receivables not past due in 2019?
{"Trade receivables not past due": {"2019": 2082, "2018": 2091}, "Trade receivables past due, net of allowance for doubtful accounts -- Under 60 days": {"2019": 541, "2018": 508}, "Trade receivables past due, net of allowance for doubtful accounts -- 60 to 120 days": {"2019": 232, "2018": 304}, "Trade receivables past due, net of allowance for doubtful accounts -- Over 120 days": {"2019": 64, "2018": 72}, "Trade receivables past due, net of allowance for doubtful accounts -- Trade receivables, net of allowance for doubtful accounts": {"2019": 2919, "2018": 2975}}
trade_receivables_not_past_due_2019 = df["Trade receivables not past due"]["2019"]
trade_receivables_not_past_due_2018 = df["Trade receivables not past due"]["2018"]
answer = trade_receivables_not_past_due_2019 - trade_receivables_not_past_due_2018