What is the change in the percentage of weighted average expected volatility from 2018 to 2019?
{"Dividend yield": {"Year ended September 30, 2019": 0.0, "Year ended September 30, 2018": 0.0}, "Weighted average expected volatility": {"Year ended September 30, 2019": 37.77, "Year ended September 30, 2018": 43.68}, "Weighted average risk-free interest rate": {"Year ended September 30, 2019": 2.92, "Year ended September 30, 2018": 2.7}, "Weighted average expected life (in years)": {"Year ended September 30, 2019": 3.0, "Year ended September 30, 2018": 3.7}, "Vesting period (in years)": {"Year ended September 30, 2019": 3.0, "Year ended September 30, 2018": 3.0}}
weighted_average_expected_volatility_2019 = df["Weighted average expected volatility"]["Year ended September 30, 2019"]
weighted_average_expected_volatility_2018 = df["Weighted average expected volatility"]["Year ended September 30, 2018"]
answer = weighted_average_expected_volatility_2019 - weighted_average_expected_volatility_2018