A new fast food restaurant opened 5 months ago. The table belows shows the number of burgers they've sold so far.
| Month | Burgers Sold |
| 1 | 1,989 |
| 2 | 3,004 |
| 3 | 1,698 |
| 4 | 2,744 |
| 5 | 2,466 |
| 11,901 |
The next month (after spending some money on an ad) they sold 8 times as many as they had sold in the previous 5 months. How many fewer burgers did they sell before running the ad than they did after running it?