What was the change in nonvested shares at expected attainment between 2018 and 2019?
{"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_2019 = df["Nonvested at December 31, 2019"]["Number of Shares at Expected Attainment"]
nonvested_2018 = df["Nonvested at December 31, 2018"]["Number of Shares at Expected Attainment"]
answer = nonvested_2019 - nonvested_2018