What is the difference in Weighted Average Grant Date Fair Value between nonvested shares and forfeited shares in 2018?
{"Nonvested at December 31, 2018": {"Number of Shares at Expected Attainment": 540697.0, "Weighted Average Grant Date Fair Value": 19.83}, "Forfeited": {"Number of Shares at Expected Attainment": -56567.0, "Weighted Average Grant Date Fair Value": 18.8}, "Change in expected attainment": {"Number of Shares at Expected Attainment": 185339.0, "Weighted Average Grant Date Fair Value": 20.09}, "Nonvested at December 31, 2019": {"Number of Shares at Expected Attainment": 669469.0, "Weighted Average Grant Date Fair Value": 20.12}}
nonvested_fair_value = df["Nonvested at December 31, 2018"]["Weighted Average Grant Date Fair Value"]
forfeited_fair_value = df["Forfeited"]["Weighted Average Grant Date Fair Value"]
answer = nonvested_fair_value - forfeited_fair_value