What was the change in the number of employees in 2019 from 2018?
{"Grant date": {"2015 Grant": 1.0, "2016 Grant": 1.0, "2017 Grant": 1.0, "2018 Grant": 1.0, "2019 Grant": 1.0}, "Exercise price": {"2015 Grant": 2797.0, "2016 Grant": 4477.3, "2017 Grant": 5496.7, "2018 Grant": 7240.0, "2019 Grant": 7835.0}, "Number of employees": {"2015 Grant": 1038.0, "2016 Grant": 1040.0, "2017 Grant": 1229.0, "2018 Grant": 1294.0, "2019 Grant": 1318.0}, "Shares under scheme": {"2015 Grant": 34449.0, "2016 Grant": 22173.0, "2017 Grant": 22411.0, "2018 Grant": 16687.0, "2019 Grant": 16820.0}, "Vesting period": {"2015 Grant": 3.0, "2016 Grant": 3.0, "2017 Grant": 3.0, "2018 Grant": 3.0, "2019 Grant": 3.0}, "Expected volatility": {"2015 Grant": 21.0, "2016 Grant": 21.0, "2017 Grant": 21.0, "2018 Grant": 19.0, "2019 Grant": 21.0}, "Risk free interest rate": {"2015 Grant": 0.4, "2016 Grant": 0.1, "2017 Grant": 0.4, "2018 Grant": 0.8, "2019 Grant": 0.5}, "Expected dividend yield": {"2015 Grant": 2.5, "2016 Grant": 2.5, "2017 Grant": 2.3, "2018 Grant": 2.0, "2019 Grant": 1.8}, "Fair value": {"2015 Grant": 2931.3, "2016 Grant": 4696.7, "2017 Grant": 5799.0, "2018 Grant": 7623.7, "2019 Grant": 8305.1}}
number_of_employees_2019_grant = df["Number of employees"]["2019 Grant"]
number_of_employees_2018_grant = df["Number of employees"]["2018 Grant"]
answer = number_of_employees_2019_grant - number_of_employees_2018_grant