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:
|  |  | Year Ended December 31, | Year Ended December 31, | Year Ended December 31, |
| --- | --- | --- | --- | --- |
|  |  | 2018 | 2017 (a) | 2016 (a) |
| Operating revenue: | Passenger revenue | $37,706 | $34,460 | $33,429 |
| Operating revenue: | Cargo | 1,237 | 1,114 | 934 |
| Operating revenue: | Other operating revenue | 2,360 | 2,210 | 2,195 |
| Operating revenue: | Total operating revenue | 41,303 | 37,784 | 36,558 |
| Operating expense: | Salaries and related costs | 11,458 | 10,941 | 10,176 |
| Operating expense: | Aircraft fuel | 9,307 | 6,913 | 5,813 |
| Operating expense: | Regional capacity purchase | 2,601 | 2,232 | 2,197 |
| Operating expense: | Landing fees and other rent | 2,359 | 2,240 | 2,165 |
| Operating expense: | Depreciation and amortization | 2,240 | 2,149 | 1,977 |
| Operating expense: | Aircraft maintenance materials and outside repairs | 1,767 | 1,856 | 1,749 |
| Operating expense: | Distribution expenses | 1,558 | 1,435 | 1,395 |
| Operating expense: | Aircraft rent | 433 | 621 | 680 |
| Operating expense: | Special charges | 487 | 176 | 745 |
| Operating expense: | Other operating expenses | 5,801 | 5,550 | 5,317 |
| Operating expense: | Total operating expense | 38,011 | 34,113 | 32,214 |
| Operating expense: | Operating income | 3,292 | 3,671 | 4,344 |
| Nonoperating income (expense): | Interest expense | (729) | (671) | (674) |
| Nonoperating income (expense): | Interest capitalized | 70 | 84 | 72 |
| Nonoperating income (expense): | Interest income | 101 | 57 | 42 |
| Nonoperating income (expense): | Miscellaneous, net | (76) | (101) | (11) |
| Nonoperating income (expense): | Total nonoperating expense, net | (634) | (631) | (571) |
| Nonoperating income (expense): | Income before income taxes | 2,658 | 3,040 | 3,773 |
| Nonoperating income (expense): | Income tax expense | 529 | 896 | 1,539 |
| Nonoperating income (expense): | Net income | $2,129 | $2,144 | $2,234 |
| Nonoperating income (expense): | Earnings per share, basic | $7.73 | $7.08 | $6.77 |
| Nonoperating income (expense): | Earnings per share, diluted | $7.70 | $7.06 | $6.76 |
Question:
What was the operating expense of United airlines on aircraft fuel in the year 2018 ?
Answer:
9,307