In the following paper (Example 2.1), it has been mentioned to K+M to provide an example of a pseudo valuation domain which is not a valuation ring, and its reference is Gilmer's book, but I have no access to Gilmer's book.

Can someone help me and explain what K+M is?

<cite authors="Hedstrom, J. R.; Houston, E. G.">_Hedstrom, J. R.; Houston, E. G._, Pseudo-valuation domains. II, Houston J. Math. 4, 199-207 (1978). [ZBL0416.13014](https://zbmath.org/?q=an:0416.13014).</cite>