What is the increase / (decrease) in the Device payment plan agreement receivables, gross from 2018 to 2019?
{"Unbilled": {"2019": 12403, "2018": 11485}, "Billed: -- Current": {"2019": 815, "2018": 641}, "Billed: -- Past due": {"2019": 262, "2018": 209}, "Billed: -- Device payment plan agreement receivables, gross": {"2019": 13480, "2018": 12335}}
device_payment_plan_agreement_receivables_gross_2019 = df["Billed: -- Device payment plan agreement receivables, gross"]["2019"]
device_payment_plan_agreement_receivables_gross_2018 = df["Billed: -- Device payment plan agreement receivables, gross"]["2018"]
answer = device_payment_plan_agreement_receivables_gross_2019 - device_payment_plan_agreement_receivables_gross_2018