Given a BST and an integer K. Find the Kth Smallest element in the BST using O(1) extra space.

Example 1:

Input:
      2
    /   \
   1     3
K = 2
Output: 2
Explanation: 2 is the 2nd smallest element in the BST

Example 2:

Input:
        2
      /  \
     1    3
K = 5
Output: -1
Explanation: There is no 5th smallest element in the BST as the size of BST is 3

Your Task:
You don't need to read input or print anything. Your task is to complete the function KthSmallestElement() which takes the root of the BST and integer K as inputs and returns the Kth smallest element in the BST, if no such element exists return -1.

Expected Time Complexity: O(N).
Expected Auxiliary Space: O(1).

Constraints:
1<=Number of nodes<=10^5