What is the average expected life over 2018 and 2019?
{"Weighted average fair value per option granted": {"2019": 2.34, "2018": 2.13}, "Weighted average share price": {"2019": 58.0, "2018": 57.0}, "Weighted average exercise price": {"2019": 58.0, "2018": 56.0}, "Expected dividend growth": {"2019": 5.0, "2018": 5.0}, "Expected volatility": {"2019": 14.0, "2018": 12.0}, "Risk-free interest rate": {"2019": 2.0, "2018": 2.0}, "Expected life (years)": {"2019": 4.0, "2018": 4.0}}
expected_life_years_2019 = df["Expected life (years)"]["2019"]
expected_life_years_2018 = df["Expected life (years)"]["2018"]
riskfree_interest_rate_2019 = df["Risk-free interest rate"]["2019"]
answer = ( expected_life_years_2019 + expected_life_years_2018 ) / riskfree_interest_rate_2019