What was the change in the Risk-free interest rate from 2017 to 2018?
{"Weighted average grant-date fair value": {"January 3, 2020": 11.89, "December 28, 2018": 13.85, "December 29, 2017": 11.53}, "Expected term (in years)": {"January 3, 2020": 4.4, "December 28, 2018": 4.7, "December 29, 2017": 4.7}, "Expected volatility": {"January 3, 2020": 24.3, "December 28, 2018": 26.6, "December 29, 2017": 29.7}, "Risk-free interest rate": {"January 3, 2020": 2.4, "December 28, 2018": 2.6, "December 29, 2017": 1.9}, "Dividend yield": {"January 3, 2020": 2.2, "December 28, 2018": 2.0, "December 29, 2017": 2.5}}
risk_free_interest_rate_2018 = df["Risk-free interest rate"]["December 28, 2018"]
risk_free_interest_rate_2017 = df["Risk-free interest rate"]["December 29, 2017"]
answer = risk_free_interest_rate_2018 - risk_free_interest_rate_2017