6. Define the sequence $\left\{a_{n}\right\}: a_{n}$ is the last digit of $1+2+\cdots+n$, and $S_{n}$ is the sum of the first $n$ terms of the sequence $\left\{a_{n}\right\}$. Then $S_{2016}=$
$\qquad$.
The sequence $\left\{a_{n}\right\}$ has a period of 20: $1,3,6,0,5,1,8,6,5,5,6,8,1,5,0,6,3,1,0,0$.
And $2016=20 \cdot 100+16, S_{20}=70, S_{16}=66$, so $S_{2016}=100 \cdot 70+66=7066$.