What is the difference in weighted-average grant date fair value between granted and forfeited 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}}
granted_fair_value = df["Granted"]["Weighted- Average Grant Date Fair Value"]
forfeited_fair_value = df["Forfeited"]["Weighted- Average Grant Date Fair Value"]
answer = granted_fair_value - forfeited_fair_value