What was the change in risk-free interest rate 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"}}
risk_free_interest_rate_2018 = df["Risk-free interest rate"]["2018"]
risk_free_interest_rate_2017 = df["Risk-free interest rate"]["2017"]
answer = risk_free_interest_rate_2018 - risk_free_interest_rate_2017