What is the change in expected life in years used in the Black-Scholes model between 2018 and 2019?
{"Weighted-average fair value of options granted per share": {"2019": 8.78, "2018": 6.63}, "Historical volatility": {"2019": 60, "2018": 46}, "Risk-free interest rate": {"2019": 2.1, "2018": 2.84}, "Dividend yield": {"2019": "\u2014", "2018": "\u2014"}, "Expected life in years": {"2019": 3.6, "2018": 5.7}}
expected_life_in_years_2019 = df["Expected life in years"]["2019"]
expected_life_in_years_2018 = df["Expected life in years"]["2018"]
answer = expected_life_in_years_2019 - expected_life_in_years_2018