Question about the degree of the composition of two isogenies

Question

I'm reading a paper about isogeny based cryptography and I came across a sentence that seems to imply that for separable isogenies $\phi$ and $\psi$, the following holds: deg$(\phi \circ \psi)=$ deg$(\phi)$deg$(\psi)$. I can't seem to figure out why this is true, or if it is even true. Any clarification would be super helpful. Thanks!