Intro to Summation (Σ) and Product (Π) Notation

Assume that we have a sequence such that, for $n \ge 2$, $a_n = a_{n-1} - 2a_{n-2}$. Also assume that $a_0$ and $a_1$ are given. In terms of $a_0$ and $a_1$, what is: $$\sum_{n=1}^4 a_n$$