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 | Gallons Consumed (in millions) | Fuel Expense       (in millions) | Average Price Per Gallon | Percentage of Total Operating Expense | Available Seat Miles per Fuel Gallon |
| --- | --- | --- | --- | --- | --- |
| 2018 | 4,137 | $9,307 | $2.25 | 24% | 67 |
| 2017 | 3,978 | $6,913 | $1.74 | 20% | 66 |
| 2016 | 3,904 | $5,813 | $1.49 | 18% | 65 |
Question:
How many gallons of fuel was consumed by United airlines in 2018 ?
Answer:
4,137