What was the change in the Risk-free interest rate from 2018 to 2019?
{"Expected term (in years)": {"2019": 0.5, "2018": 0.5}, "Risk-free interest rate": {"2019": 2.3, "2018": 1.4}, "Expected volatility": {"2019": 39.1, "2018": 29.9}, "Expected dividend yield  ": {"2019": "\u2014%", "2018": "\u2014%"}, "Grant date fair value per ordinary share": {"2019": 36.69, "2018": 27.15}}
risk_free_interest_rate_2019 = df["Risk-free interest rate"]["2019"]
risk_free_interest_rate_2018 = df["Risk-free interest rate"]["2018"]
answer = risk_free_interest_rate_2019 - risk_free_interest_rate_2018