I've tried to use Lalescu Liviu's method described on the webboard, but my implementation has some minor bug I can't find. Can someone please help me by sending me ac program in pascal? In return, I can send some of my ac-ed programs...

I had same problem. Maybe, you should pay attention to binomial coeficients' computation method (check long integers for overflow).

I also had this problem with the binomial coefficients, because I was using the formula with factorials and long long.

It shows up under this test:
1 2000000000 15 2
answer:
286658146