5. Given the sequence $\left\{a_{n}\right\}$, the first two terms are $a_{1}=2, a_{2}=4$. Let $b_{n}=a_{n+1}-a_{n}$. If the sequence $\left\{b_{n}\right\}$ is an arithmetic sequence with a common difference of 1, then $a_{2020}=$ $\qquad$
Reference answer: It is easy to know that $b_{1}=a_{2}-a_{1}=2$, since the common difference is 1, so $b_{n}=n+1$. Therefore, $a_{2020}=a_{1}+\sum^{2019} b_{i}=2+2+3+\cdots+2020=2041211$.