Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Employee<br>Group | Number of Employees | Union | Agreement Open<br>for Amendment |
| --- | --- | --- | --- |
| Flight Attendants | 22,676 | Association of Flight Attendants (the “AFA”) | August 2021 |
| Passenger Service | 13,299 | International Association of Machinists and Aerospace Workers (the “IAM”) | December 2021 |
| Fleet Service | 13,187 | IAM | December 2021 |
| Pilots | 11,492 | Air Line Pilots Association, International | January 2019 |
| Technicians and Related & Flight Simulator Technicians | 9,535 | International Brotherhood of Teamsters (the “IBT”) | December 2022 |
| Storekeeper Employees | 1,000 | IAM | December 2021 |
| Dispatchers | 402 | Professional Airline Flight Control Association | December 2021 |
| Fleet Tech Instructors | 111 | IAM | December 2021 |
| Load Planners | 71 | IAM | December 2021 |
| Security Officers | 51 | IAM | December 2021 |
| Maintenance Instructors | 40 | IAM | December 2021 |
Question:
As of December 2017, how many people worked for United Airlines as security officers?
Answer:
51