What is the difference in grant date fair value between the vested stocks and the forfeited stocks?
{"Outstanding at January 1, 2019   ": {"Number of Units": 997173.0, "Grant Date Fair Value": 52.22}, "Granted   ": {"Number of Units": 945159.0, "Grant Date Fair Value": 49.48}, "Vested   ": {"Number of Units": -386060.0, "Grant Date Fair Value": 51.79}, "Forfeited   ": {"Number of Units": -59579.0, "Grant Date Fair Value": 50.56}, "Outstanding at December 31, 2019   ": {"Number of Units": 1496693.0, "Grant Date Fair Value": 50.67}}
vested_grant_date_fair_value = df["Vested   "]["Grant Date Fair Value"]
forfeited_grant_date_fair_value = df["Forfeited   "]["Grant Date Fair Value"]
answer = vested_grant_date_fair_value - forfeited_grant_date_fair_value