What was the percentage change in the Fair value of options vested between 2018 and 2019?
{"Intrinsic value of exercises": {"April 26, 2019": 31, "April 27, 2018": 37, "April 28, 2017": 26}, "Proceeds received from exercises": {"April 26, 2019": 25, "April 27, 2018": 88, "April 28, 2017": 60}, "Fair value of options vested": {"April 26, 2019": 2, "April 27, 2018": 8, "April 28, 2017": 15}}
fair_value_2019 = df["Fair value of options vested"]["April 26, 2019"]
fair_value_2018 = df["Fair value of options vested"]["April 27, 2018"]
answer = (fair_value_2019 - fair_value_2018) / fair_value_2018 * 100.0