The positive value of semaphore shows the number of successful down() that can be performed on the semaphore. Here 6 successful down() can be performed after that semaphore value will become 4. 1 down() decreases the value of semaphore by 1.
The up() on semaphore increases it's value by 1 & it is always successful. After 4 up() final value of semaphore become 8.