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

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