What is the change in the Shares available for grant under stock plans from 2018 to 2019?
{"Exercise of stock options to purchase common stock": {"July 31, 2019 ": 216727, "July 31, 2018": 537064}, "Vesting of restricted stock awards": {"July 31, 2019 ": 2384673, "July 31, 2018": 2932155}, "Shares available for grant under stock plans": {"July 31, 2019 ": 24776361, "July 31, 2018": 21592494}, "Total common stock reserved for issuance": {"July 31, 2019 ": 27377761, "July 31, 2018": 25061713}}
shares_2019 = df["Shares available for grant under stock plans"]["July 31, 2019 "]
shares_2018 = df["Shares available for grant under stock plans"]["July 31, 2018"]
answer = shares_2019 - shares_2018