What is the change in Government segment: Billed between December 31, 2018 and 2019?
{"Government segment: -- Billed": {"2019": 11608, "2018": 9100}, "Government segment: -- Advanced billings": {"2019": -608, "2018": -563}, "Government segment: -- ": {"2019": 11000, "2018": 8537}, "Restaurant/Retail segment: -- Accounts receivable - net": {"2019": 30774, "2018": 17682}, "Restaurant/Retail segment: -- ": {"2019": 41774, "2018": 26219}}
govt_billed_2019 = df["Government segment: -- Billed"]["2019"]
govt_billed_2018 = df["Government segment: -- Billed"]["2018"]
answer = govt_billed_2019 - govt_billed_2018