# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def findSecondMinimumValue(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        res = self.findSecondMinimumValues(root)
        return res if res != float('inf') else -1
    def findSecondMinimumValues(self, root, min1 = float('inf'), min2 = float('inf')):
        if not root: return min2
        min1 = min(root.val, min1)
        if min1 < root.val < min2: min2 = root.val
        return min(self.findSecondMinimumValues(root.left, min1, min2),
                   self.findSecondMinimumValues(root.right, min1, min2))