Given the `root` of a binary tree, return the sum of values of nodes with an even-valued grandparent. If there are no nodes with an even-valued grandparent, return `0`.

A grandparent of a node is the parent of its parent if it exists.

Example 1:

    Input: root = [6,7,8,2,7,1,3,9,null,1,4,null,null,null,5]
    Output: 18
    Explanation: The red nodes are the nodes with even-value grandparent while the blue nodes are the even-value grandparents.

Example 2:

    Input: root = [1]
    Output: 0

Constraints:

- The number of nodes in the tree is in the range `[1, 104]`.
- `1 <= Node.val <= 100`