Given a BST and an integer K. Find the Kth Smallest element in the BST. 

Example 1:

Input:
      2
    /   \
   1     3
K = 2
Output: 2

Example 2:

Input:
        2
      /  \
     1    3
K = 5
Output: -1

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 return 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<=100000