What is the difference in weighted-average grant date fair value for unvested shares in 2018 and vested shares?
{"Unvested shares at December 31, 2018": {"Number of Shares": 4117.0, "Weighted- Average Grant Date Fair Value": 41.94}, "Granted": {"Number of Shares": 1589.0, "Weighted- Average Grant Date Fair Value": 55.69}, "Forfeited": {"Number of Shares": -510.0, "Weighted- Average Grant Date Fair Value": 45.72}, "Vested": {"Number of Shares": -1440.0, "Weighted- Average Grant Date Fair Value": 40.61}, "Unvested shares at December 31, 2019": {"Number of Shares": 3756.0, "Weighted- Average Grant Date Fair Value": 47.76}}
unvested_fair_value = df["Unvested shares at December 31, 2018"]["Weighted- Average Grant Date Fair Value"]
vested_fair_value = df["Vested"]["Weighted- Average Grant Date Fair Value"]
answer = ( unvested_fair_value - vested_fair_value )