What is the increase / (decrease) in the unbilled 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}}
unbilled_2019 = df["Unbilled"]["2019"]
unbilled_2018 = df["Unbilled"]["2018"]
answer = unbilled_2019 - unbilled_2018