What was the percentage change in expected life (years) between 2018 and 2019?
{"Expected life (years)": {"2019": 2.8, "2018": 2.9, "2017": 2.9}, "Interest rate": {"2019": 2.5, "2018": 2.4, "2017": 1.5}, "Volatility": {"2019": 29.3, "2018": 28.0, "2017": 26.5}, "Expected dividend yield": {"2019": "\u2014", "2018": "\u2014", "2017": "\u2014"}}
expected_life_2019 = df["Expected life (years)"]["2019"]
expected_life_2018 = df["Expected life (years)"]["2018"]
answer = ( expected_life_2019 - expected_life_2018 ) / expected_life_2018 * 100.0