18. Let $f(x)$ be the polynomial $\left(x-a_{1}\right)\left(x-a_{2}\right)\left(x-a_{3}\right)\left(x-a_{4}\right)\left(x-a_{5}\right)$ where $a_{1}, a_{2}, a_{3}, a_{4}$ and $a_{5}$ are distinct integers. Given that $f(104)=2012$, evaluate $a_{1}+a_{2}+a_{3}+a_{4}+a_{5}$.