What was the change in Unvested RSUs and restricted stock outstanding between 2018 and 2019?
{"Unvested PSUs, RSUs and restricted stock outstanding, December 31, 2018": {"Number of Shares": 1570.0, "Weighted Average Grant Date Fair Value": 18.52}, "PSUs, RSUs and restricted stock granted": {"Number of Shares": 897.0, "Weighted Average Grant Date Fair Value": 9.63}, "PSUs, RSUs and restricted stock vested": {"Number of Shares": -368.0, "Weighted Average Grant Date Fair Value": 17.23}, "PSUs, RSUs and restricted stock forfeited": {"Number of Shares": -208.0, "Weighted Average Grant Date Fair Value": 18.24}, "Unvested RSUs and restricted stock outstanding, December 31, 2019": {"Number of Shares": 1891.0, "Weighted Average Grant Date Fair Value": 14.58}}
unvested_rsu_2019 = df["Unvested RSUs and restricted stock outstanding, December 31, 2019"]["Number of Shares"]
unvested_rsu_2018 = df["Unvested PSUs, RSUs and restricted stock outstanding, December 31, 2018"]["Number of Shares"]
answer = unvested_rsu_2019 - unvested_rsu_2018