What is the change in nonvested shares between 2018 and 2019?
{"Nonvested at December 31, 2018": {"Number of Shares": 213337.0, "Weighted Average Grant Date Fair Value": 20.21}, "Vested": {"Number of Shares": -106610.0, "Weighted Average Grant Date Fair Value": 20.17}, "Forfeited": {"Number of Shares": -13885.0, "Weighted Average Grant Date Fair Value": 20.64}, "Nonvested at December 31, 2019": {"Number of Shares": 92842.0, "Weighted Average Grant Date Fair Value": 20.13}}
nonvested_at_december_31_2019_number_of_shares = df["Nonvested at December 31, 2019"]["Number of Shares"]
nonvested_at_december_31_2018_number_of_shares = df["Nonvested at December 31, 2018"]["Number of Shares"]
answer = nonvested_at_december_31_2019_number_of_shares - nonvested_at_december_31_2018_number_of_shares