Here is my submission:

http://codeforces.net/contest/670/submission/17734251

I get a RUNTIME_ERROR for the 13th test case and I have no idea why. I cannot debug it, since the input is truncated. I tried to reproduce a test case with similar n and k, but random id's and it is working fine, or at least it doesn't cause a runtime error.

I hope that such questions are appropriate for the blog and I would very much appreciate your help. I would also be very happy if you could give me some general tips on how to approach such problems (figuring out why a truncated input caused a runtime error).