NZ-NZMO 2021 Problem 7

Let $a, b, c, d$ be integers such that $a > b > c > d \geq -2021$ and

$$\frac{a + b}{b + c} = \frac{c + d}{d + a}$$

(and $b + c \neq 0 \neq d + a$ ). What is the maximum possible value of $ac$ ?