To determine $S_{18}$ (the sum of the first 18 terms) of a geometric series, we need some specific information:

1. The first term $a$.
2. The common ratio $r$.
3. The number of terms $n$ (which is 18 in this case).

The formula for the sum of the first $n$ terms of a geometric series is:

$S_n = a \frac{1 - r^n}{1 - r} \quad \text{(for } r \neq 1\text{)}$

Where:

• $S_n$ is the sum of the first $n$ terms.
• $a$ is the first term.
• $r$ is the common ratio.
• $n$ is the number of terms.

If you provide the values for $a$ and $r$, I can compute $S_{18}$ for you.