Let n be an integer greater than 2, and let the largest prime divisor of n be denoted by $\tilde{n}$.\\
The terms of the sequence $(a_n)$ are defined for $n \geq 2$ as
$$a _ { n } = \left\{ \begin{aligned}
1 & , \tilde{n} < 10 \\
- 1 & , \tilde{n} > 10
\end{aligned} \right.$$
Accordingly, what is the sum $\sum _ { n = 15 } ^ { 30 } a _ { n }$?\\
A) 2\\
B) 3\\
C) 4\\
D) 5\\
E) 6