What is the change in Assets: Voyage receivables from Balance at December 31, 2017 to January 1, 2018?
{"Assets -- Voyage receivables": {"Balance at December 31, 2017": 24209, "Adjustments Due to ASC 606": 1336, "Balance at January 1, 2018": 25545}, "Liabilities -- Deferred income taxes": {"Balance at December 31, 2017": 83671, "Adjustments Due to ASC 606": -108, "Balance at January 1, 2018": 83563}, "Equity -- Accumulated deficit": {"Balance at December 31, 2017": -265758, "Adjustments Due to ASC 606": -1228, "Balance at January 1, 2018": -266986}}
voyage_receivables_balance_2018 = df["Assets -- Voyage receivables"]["Balance at January 1, 2018"]
voyage_receivables_balance_2017 = df["Assets -- Voyage receivables"]["Balance at December 31, 2017"]
answer = voyage_receivables_balance_2018 - voyage_receivables_balance_2017