Given that the task is very specific I'd say it is, as it is performant and the code is readable.

If you had to, for example, generate neighbors up to a variable distance n received as a parameter of the function, that would be a different story.

cool~

Note that 1^3 + ... + n^3 = (1+ ... + n)^2 is not true

1^3 + ... + n^3 is 1/4 n^2 (1 + n)^2

Yep, thank you. It's right.

You should check the result when you have finished computing - this code doesn't return -1 if m=4.

You can to add simple test case "()(" - is invalid, for C++. Without it, I committed invalid solution. =)