What was the change in the Exercisable amount in 2019 from 2018?
{"Outstanding": {"2019": 4.2, "2018": 3.6, "2017": 4.1}, "Vested and expected to vest": {"2019": 5.0, "2018": 3.6, "2017": 4.1}, "Exercisable": {"2019": 2.1, "2018": 2.4, "2017": 2.8}}
exercisable_2019 = df["Exercisable"]["2019"]
exercisable_2018 = df["Exercisable"]["2018"]
answer = exercisable_2019 - exercisable_2018