VTMO 2016 — Grade 6 — semifinal Question 7

The sequence is $1 , 2 , 3 , 0 , 5 , - 2 , 7 , - 4 , \ldots$, with $a_{n} = a$_$\left(n - 3\right) + a$_$\left(n - 2\right) - a$_$\left(n - 1\right)$. Find the sum of its first $2016$ terms.