Why $\mathcal{O}(n^2)$ multiplication of coefficient required for canonical form of polynomial?

I was going to a textbook and am not able to understand the following:

Let $ F(x)$ is given as a product $ F(x) = \sum_{i=0}^{n} (x – a_i)$ . Transforming $ F(x)$ to its canonical form by consecutively multiplying the $ i$ th monomial with the product of the first $ i-1$ monomials requires $ \mathcal{O}(n^2)$ multiplications of coefficients.

A canonical form of polynomial is $ \sum_{i=0}^{n} c_i x_i$ .

Why $ \mathcal{O}(n^2)$ ?