Sum raw with the same continuous flags

[Shenli Zhu <zhushenli@xxxxxxxxx>]

Hi, there are 2 columns, flag(boolean) and num(integer),in a table.
Table is like
| flag | num |  
|------+-----|
|    1 |   2 | \ 5
|    1 |   3 | /
|    0 |   1 | \ 7
|    0 |   6 | /
|    1 |   4 | \ 9
|    1 |   5 | /
|  ... | ... |  
I want to sum up the raw with the same continuous flags. E.g. flag in 1st and
2nd row are both 1, 3rd and 4th are both 0, 5th and 6th are both 1. So
the table becomes
| flag | num |
|------+-----|
|    1 |   5 |
|    0 |   7 |
|    1 |   9 |

Can I do this in SQL or PL/pgSQL? Any suggestions are welcome.