I think, that there are some bugs in tests answers and judge
solutions. My long ariphmetics with bugs gets AC and without - WA.
All problems were in multiplication. I had overflow of WORD and got
AC. Who knows, how to post my opinion to site Creators.

you can use an array of integers for that huge number . when
n = 1800 and k = 10 the answer have about 3600 digits !!! and
you cant store it in nor long nor any other variable.
in this problem you must find a recursive relations . ( not to
consider all answer ). it has a nice soloution . if you use
this method but not receive the correct answer mail me at :
ioi_khafan@hotmail.com . thanks . bye