What was the percentage change in Expected dividend yield between 2017 and 2018?
{"Expected dividend yield": {"2019": 0.6, "2018": 0.6, "2017": 0.8}, "Risk-free interest rate": {"2019": 2.3, "2018": 1.4, "2017": 0.5}, "Expected volatility(1)": {"2019": 28.6, "2018": 23.0, "2017": 33.0}, "Expected life": {"2019": 0.5, "2018": 0.5, "2017": 0.5}}
expected_dividend_yield_2019 = df["Expected dividend yield"]["2019"]
expected_dividend_yield_2017 = df["Expected dividend yield"]["2017"]
answer = expected_dividend_yield_2019 - expected_dividend_yield_2017