How much did the Expected volatility change by between 2018 and 2019?
{"Weighted average share price at the grant date": {"2019": 373.33, "2018": 405.0}, "Risk free rate": {"2019": 1.08, "2018": 1.77}, "Dividend yield": {"2019": 0.23, "2018": 0.24}, "Expected volatility (Note)": {"2019": 30.0, "2018": 30.0}}
expected_volatility_note_2019 = df["Expected volatility (Note)"]["2019"]
expected_volatility_note_2018 = df["Expected volatility (Note)"]["2018"]
answer = expected_volatility_note_2019 - expected_volatility_note_2018