What was the change in Intrinsic value of exercises between 2017 and 2018?
{"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}}
intrinsic_value_2018 = df["Intrinsic value of exercises"]["April 27, 2018"]
intrinsic_value_2017 = df["Intrinsic value of exercises"]["April 28, 2017"]
answer = intrinsic_value_2018 - intrinsic_value_2017