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?

Hedstrom, J. R.; Houston, E. G., Pseudo-valuation domains. II, Houston J. Math. 4, 199-207 (1978). ZBL0416.13014.