What was the increase / (decrease) in the unbilled from 2018 to 2019?
{"Unbilled": {"2019": 22827, "2018": 24282}, "Billed: -- Current": {"2019": 1286, "2018": 1465}, "Billed: -- Past due": {"2019": 236, "2018": 271}, "Billed: -- Device payment plan agreement receivables, gross": {"2019": 24349, "2018": 26018}}
unbilled_2019 = df["Unbilled"]["2019"]
unbilled_2018 = df["Unbilled"]["2018"]
answer = unbilled_2019 - unbilled_2018