Hi reader. Coming straight to the point. The value of (1ll<<59) or 2^59 is 576460752303423488(say x).So if you do log2(x),the answer should come out to be 59.That's indeed the case. But here is the catch... .If you do log2(x-1) in C++ 14(not sure for other versions),it also gives 59 as output..(but actual answer is 58), which is wrong and that caused me a fst in recent Div 3 problem F.

So, can anyone explain why is this happening and how to avoid these types of things. Thanks for spending your time in reading this blog.