What was the change in the number of shares under scheme under the 2019 Grant from the 2018 Grant?
{"Grant date": {"2015 Grant": 11.0, "2016 Grant": 5.0, "2017 Grant": 26.0, "2018 Grant": 4.0, "2019 Grant": 15.0}, "Mid market share price at grant date": {"2015 Grant": 3460.0, "2016 Grant": 3550.0, "2017 Grant": 5256.0, "2018 Grant": 5560.0, "2019 Grant": 8161.0}, "Number of employees": {"2015 Grant": 15.0, "2016 Grant": 13.0, "2017 Grant": 12.0, "2018 Grant": 12.0, "2019 Grant": 12.0}, "Shares under scheme": {"2015 Grant": 70290.0, "2016 Grant": 69890.0, "2017 Grant": 62356.0, "2018 Grant": 60899.0, "2019 Grant": 60626.0}, "Vesting period": {"2015 Grant": 3.0, "2016 Grant": 3.0, "2017 Grant": 3.0, "2018 Grant": 3.0, "2019 Grant": 3.0}, "Probability of vesting": {"2015 Grant": 71.5, "2016 Grant": 70.8, "2017 Grant": 73.1, "2018 Grant": 73.5, "2019 Grant": 74.1}, "Fair value": {"2015 Grant": 2473.9, "2016 Grant": 2513.4, "2017 Grant": 3842.1, "2018 Grant": 4084.4, "2019 Grant": 6048.9}}
shares_2019 = df["Shares under scheme"]["2019 Grant"]
shares_2018 = df["Shares under scheme"]["2018 Grant"]
answer = shares_2019 - shares_2018