Prefer using a ternary operator, it increases readability !

return (sum % n == 0) ? sum / n : -1;

It means that we got two cases for our return value :

if sum % n == 0, so that sum is divisible by n, we can return the result of the integer division of sum by n. Otherwise, we return -1. It's a common syntax, try to get used to it :D

Great solution ! But using a stack is better I guess.

EDIT : You can also use basic mathematical operations so you keep using an integer only. Take a look at my solution, we have the same approach :D

Can you please explain more ?

You can use one variable instead of two. Just accumulate everything in a variable called sum without distinction between left and right.