Partially correct paradox

In some problems, the logic I apply seems to be true. And it works fine with a sub task. But with larger values [ original constraints ] the sub task shows a WA output.
Suppose that a constraint is as large as 10^10, then how do we approach a correct solution?

Besides we aren’t given any reference test cases.

It might be frustrating (very very) but finding out the cases where your code fails gives an immense pleasure. Just keep trying . think about the corner cases bassed on constraints,etc.