Help regarding spoj problem ALIENS

Due to the time limit in the question, this question is to be done in O(n), but I can’t find any problem in my code, but its giving WA there. I have tried many test cases on my own. I am new to programming, so couldn’t understand the sliding window algo written in the comments.
Thanks in advance!

Here is my solution → bg4CIF - Online C Compiler & Debugging Tool - Ideone.com