What is the change in the dividend yield from 2018 to 2019?
{"Expected volatility": {"June 30, 2019": 32.65, "June 24, 2018": 34.07, "June 25, 2017": 27.48}, "Risk-free interest rate": {"June 30, 2019": 2.52, "June 24, 2018": 2.35, "June 25, 2017": 1.55}, "Expected term (years)": {"June 30, 2019": 2.92, "June 24, 2018": 2.92, "June 25, 2017": 2.92}, "Dividend yield": {"June 30, 2019": 2.49, "June 24, 2018": 1.05, "June 25, 2017": 1.5}}
dividend_yield_june_30_2019 = df["Dividend yield"]["June 30, 2019"]
dividend_yield_june_24_2018 = df["Dividend yield"]["June 24, 2018"]
answer = dividend_yield_june_30_2019 - dividend_yield_june_24_2018