If you stick to multiplication and addition then the beauty of modular arithmetic ensures that over and underflow are only a problem if your final result under/overflows.

In particular you can solve the problem in this article by just subtracting the numbers from 1+2+...+n-1+n. The remainder will be your missing number (though be careful with the 1+2+...+n-1+n = n(n-1)/2 formula; division by 2 is very much not compatible with arithmetic modulo a power of 2, so divide first then multiply, or keep a running total if you want to be flexible).

Worse. The zero problem does not exist if the two variables are at different memory locations. The overflow problem of this solution always exists. Plus, if a and b are at the same memory location, it suffers from the same zero problem.