Maintain the First Value That Appears Exactly Once

Design a data structure that maintains a stream of integers and returns the first value that appears exactly once.

Implement:

class FirstUniqueValue:
 def __init__(self, nums: list[int]): pass
 def show_first_unique(self) -> int: pass
 def add(self, value: int) -> None: pass

Return -1 if no value is unique.