What was the change in unbilled receivables between 2018 and 2019?
{"Amounts billed, net": {"December 31, 2019": 227528, "December 31, 2018": 80709}, "Unbilled receivables": {"December 31, 2019": 19036, "December 31, 2018": 19733}, "Total receivables, net": {"December 31, 2019": 246564, "December 31, 2018": 100442}}
unbilled_receivables_2019 = df["Unbilled receivables"]["December 31, 2019"]
unbilled_receivables_2018 = df["Unbilled receivables"]["December 31, 2018"]
answer = unbilled_receivables_2019 - unbilled_receivables_2018