just check your program on these tests
----------
4 3
1 5 8 10
answer:
1 8
5 10
wrong answer:
1 5
8
10
----------
4 2
3 5 7 10
answer:
5 10
3 7
wrong answer:
7 10
5
3

That's helped me with wa#9:
9 0
1 1 6 6 23 35 36 38 40

Right answer has 2 trips.