class Solution:
    def pairSum(self, head: Optional[ListNode]) -> int:
        slow, fast = head, head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
            
        prev, curr = None, slow
        while curr:
            next_node = curr.next
            curr.next = prev
            prev = curr
            curr = next_node
            
        max_sum = 0
        first_half, second_half = head, prev
        while second_half:
            max_sum = max(max_sum, first_half.val + second_half.val)
            first_half = first_half.next
            second_half = second_half.next
            
        return max_sum