What was the change in the Risk-free interest rate from 2018 to 2019?
{"Expected term (in years)": {"2019": 6.1, "2018": 6.1, "2017": 6.1}, "Risk-free interest rate": {"2019": 2.7, "2018": 2.2, "2017": 2.1}, "Expected volatility": {"2019": 41.5, "2018": 39.8, "2017": 41.0}, "Expected dividend yield  ": {"2019": "\u2014%", "2018": "\u2014%", "2017": "\u2014%"}, "Estimated grant date fair value per ordinary share": {"2019": 37.15, "2018": 26.52, "2017": 20.22}}
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