What was the change in risk free interest rate between 2017 and 2018?
{"Expected life (in years)": {"2019": 7.85, "2018": 7.64, "2017": 7.01}, "Volatility": {"2019": 30.0, "2018": 32.37, "2017": 35.0}, "Risk free interest rate": {"2019": 1.9, "2018": 3.1, "2017": 2.14}, "Dividend yield": {"2019": 0.76, "2018": 0.61, "2017": 0.5}, "Weighted-average grant date fair value": {"2019": 17.12, "2018": 21.03, "2017": 21.11}, "Stock price volatility range: -- Low": {"2019": 30.0, "2018": 31.72, "2017": 28.19}, "Stock price volatility range: -- High": {"2019": 38.17, "2018": 36.73, "2017": 35.0}}
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