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 (1) (in millions) | Cost (1)(2)                  (in millions) | Average Price Per Gallon (1)(2) | Percentage of Total Operating Expense (1)(2) |
| --- | --- | --- | --- | --- |
| 2018 | 4,113 | $9,020 | $2.20 | 23.0% |
| 2017 | 4,032 | $6,756 | $1.68 | 19.2% |
| 2016 | 4,016 | $5,985 | $1.49 | 18.3% |
Question:
As of 2018, how much gallons of fuel were consumed by Delta Airlines?
Answer:
4,113