Reccurence relation???

I have a sequence $a_1,a_2,a_3...$ with $a_i> 0$ for every $i\geq 0$ . Let $a_0=1$ and $a_{n}^{2}=a_{n-1}^{2}+4n(n+1)(n+2)$ for $n\geq 1$ . If $S_n=a_1+a_2+a_3+...+a_n$ . Find $S_{20}$ .