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:
|  | Total | Owned | Leased | Seats in Standard Configuration | Average Age (In Years) |
| --- | --- | --- | --- | --- | --- |
| 777-300ER | 14 | 14 | — | 366 | 0.7 |
| 777-200ER | 55 | 40 | 15 | 267-269 | 17.8 |
| 777-200 | 19 | 19 | — | 364 | 20.5 |
| 787-9 | 21 | 21 | — | 252 | 2.1 |
| 787-8 | 12 | 12 | — | 219 | 4.5 |
| 767-400ER | 16 | 14 | 2 | 242 | 16.3 |
| 767-300ER | 35 | 22 | 13 | 183-214 | 22.5 |
| 757-300 | 21 | 9 | 12 | 213 | 15.3 |
| 757-200 | 56 | 50 | 6 | 142-169 | 21.7 |
| 737-900ER | 136 | 136 | — | 179 | 5.0 |
| 737-900 | 12 | 8 | 4 | 179 | 16.3 |
| 737-800 | 141 | 77 | 64 | 154-166 | 13.8 |
| 737-700 | 40 | 20 | 20 | 118-126 | 18.8 |
| A320-200 | 99 | 66 | 33 | 150 | 19.3 |
| A319-100 | 67 | 50 | 17 | 128 | 16.7 |
| Total mainline | 744 | 558 | 186 |  | 14.3 |
Question:
How many 777-200ER are leased by United airlines by the end of 2017?
Answer:
15