Rightmost unique value

You are given a list of positive integers. Return the last value in the list that appears exactly once in the entire list, that is, among all values with a total count of 1, the one whose position is furthest to the right. If every value occurs two or more times, or the list is empty, return -1. For example, in [4, 7, 4, 9, 7, 2] both 9 and 2 appear once, and 2 sits further right, so the answer is 2.