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

You are given an integer array nums and an integer k. Find the largest even sum of any subsequence of nums that has a length of k.
Return this sum, or -1 if such a sum does not exist.
A subsequence is an array that can be derived from another array by deleting some or no elements without changing the order of the remaining elements.
 
Please complete the following python code precisely:
```python
class Solution:
    def largestEvenSum(self, nums: List[int], k: int) -> int:
```