What is the change in expected volatility between 2018 and 2019?
{"": {"2019": 19.0, "2018": 18.0}, "Grant date (1)": {"2019": 30.0, "2018": 31.0}, "Performance period start date": {"2019": 1.0, "2018": 1.0}, "Exercise date": {"2019": 1.0, "2018": 1.0}, "Expected volatility (2)": {"2019": 15.0, "2018": 16.0}, "Expected dividend yield": {"2019": 4.0, "2018": 4.0}, "Risk-free interest rate": {"2019": 2.1, "2018": 1.9}, "Weighted average fair value at grant date": {"2019": 24.63, "2018": 20.23}}
expected_volatility_2_2018 = df["Expected volatility (2)"]["2018"]
expected_volatility_2_2019 = df["Expected volatility (2)"]["2019"]
answer = expected_volatility_2_2018 - expected_volatility_2_2019