calculus - Let $f(x + y) = f(x) + f(y) - 2f(xy)$. What is $f(x)$?

Let $f$ is a function and not constant.Suppose $x,y\in \mathbb{R}$ and
$$f(x + y) = f(x) + f(y) - 2f(xy)$$

What is $f(x)$?