Please solve the programming task below using a self-contained code snippet in a markdown code block.

You are given an array nums of non-negative integers. nums is considered special if there exists a number x such that there are exactly x numbers in nums that are greater than or equal to x.
Notice that x does not have to be an element in nums.
Return x if the array is special, otherwise, return -1. It can be proven that if nums is special, the value for x is unique.
 
Please complete the following python code precisely:
```python
class Solution:
    def specialArray(self, nums: List[int]) -> int:
```