Every finite generated module with finite free resolution is UFD

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The following is exercise 2.2.27 in Bruns and Herzog's book Cohen-Macaulay rings:

Let $R$ be a Noetherian ring such that every finite generated module has a finite free resolution. Then $R$ is UFD.

I don't know how to show that $R$ is integral domain. If this is true, using the proof of "regular local ring is UFD" in this book. Then the result follows.

Thank you in advance.