What is the change in the number of unvested RSUs between 2018 and 2019?
{"Unvested RSUs at December 31, 2018": {"Number of shares": 10623.0, "Weighted-Average Grant Date Fair Value": 40.39}, "Granted": {"Number of shares": 4426.0, "Weighted-Average Grant Date Fair Value": 45.55}, "Vested": {"Number of shares": -2758.0, "Weighted-Average Grant Date Fair Value": 47.86}, "Forfeited": {"Number of shares": -2963.0, "Weighted-Average Grant Date Fair Value": 54.61}, "Unvested RSUs at December 31, 2019": {"Number of shares": 9328.0, "Weighted-Average Grant Date Fair Value": 32.6}}
unvested_rsu_2019 = df["Unvested RSUs at December 31, 2019"]["Number of shares"]
unvested_rsu_2018 = df["Unvested RSUs at December 31, 2018"]["Number of shares"]
answer = unvested_rsu_2019 - unvested_rsu_2018