The computer centre (CC) in an institute manages the central server for which researchers queue to run their computer programs. CC processes programs only one at a time. Four computer programs are available at 10:00 am on a given day. The time required to run each program on the central server and their respective promised time of completion (i.e., due date) on that same day are given in the table.
| Program | 1 | 2 | 3 | 4 |
|---|
| Processing time (in minutes) | 60 | 40 | 90 | 100 |
| Promised time (i.e., due date) | 12 noon | 11:30 am | 1:00 pm | 1:30 pm |
If the CC follows shortest processing time (SPT) rule to sequence the computer programs, then number of tardy jobs is ______.
Note: Ignore program change-over times.