What was the change in the outstanding from 2018 to 2019?
{"": {"Number of Shares": "(in thousands)", "Weighted-Average Grant-Date": "Fair Value per Share"}, "Outstanding at December 31, 2018": {"Number of Shares": 3263.0, "Weighted-Average Grant-Date": 20.23}, "Granted": {"Number of Shares": 1580.0, "Weighted-Average Grant-Date": 23.23}, "Vested": {"Number of Shares": -1541.0, "Weighted-Average Grant-Date": 20.16}, "Canceled": {"Number of Shares": -378.0, "Weighted-Average Grant-Date": 21.52}, "Outstanding at December 31, 2019": {"Number of Shares": 2924.0, "Weighted-Average Grant-Date": 21.72}}
outstanding_at_december_31_2019_number_of_shares = df["Outstanding at December 31, 2019"]["Number of Shares"]
outstanding_at_december_31_2018_number_of_shares = df["Outstanding at December 31, 2018"]["Number of Shares"]
answer = outstanding_at_december_31_2019_number_of_shares - outstanding_at_december_31_2018_number_of_shares