What is the change in the outstanding number of shares between the beginning and end of the year 2019?
{"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}}
outstanding_2019_end = df["Outstanding at December 31, 2019   "]["Number of Units"]
outstanding_2019_start = df["Outstanding at January 1, 2019   "]["Number of Units"]
answer = outstanding_2019_end - outstanding_2019_start