COMC 2021 B Problem 2

When the product

\left(2021 x^{2021}+2020 x^{2020}+\cdots+3 x^{3}+2 x^{2}+x\right)\left(x^{2021}-x^{2020}+\cdots+x^{3}-x^{2}+x-1\right)

is expanded and simplified, what is the coefficient of x^{2021} ?