strange runtime error for I/O (on LOJ-1162)

I was solving this problem. I submitted this code[edited] and it got RTE. I couldn't find any error of my code. Luckily, I removed "IOS" from line 44 (like this) and used scanf/prinrf instead, then submitted again. Then it got AC! I was using cin/cout for I/O, so I almost always use that line("IOS"). But, is it safe? What's actually going wrong here?

Thanks!