Participated in the recent div4 contest. Got stuck in this problem CF 1003 div 4 C2 during the contest. The editorial used a left to right approach, I have used a meet at middle approach. What have I missed? I also learnt the modified binary search after the contest.

My code

It's not that I want to shove off my load to someone else, I can't access the problematic test case. Would appreciate help.