def __get_depth(root):
    """
    return 0 if unbalanced else depth + 1
    """
    if root is None:
        return 0
    left  = __get_depth(root.left)
    right = __get_depth(root.right)
    if abs(left-right) > 1 or -1 in [left, right]:
        return -1
    return 1 + max(left, right)