What was the change in Expected volatility between 2017 and 2018?
{"Expected life (years)": {"2018": 5.6, "2017": 5.6}, "Risk-free interest rate": {"2018": 2.7, "2017": 1.9}, "Expected volatility": {"2018": 26.4, "2017": 29.4}, "Expected dividend yield": {"2018": "\u2014", "2017": "\u2014"}}
expected_volatility_2018 = df["Expected volatility"]["2018"]
expected_volatility_2017 = df["Expected volatility"]["2017"]
answer = expected_volatility_2018 - expected_volatility_2017