input: Write a python function to find the maximum pair sum from a list of integers.
  assert maxPairSum([1, 2, 3, 4, 5]) == 9
  assert maxPairSum([10, 20, 30, 40, 50]) == 90
  assert maxPairSum([-10, -20, 5, 10, 15]) == 25
output: def maxPairSum(nums):
      if len(nums) < 2:
          return None
      nums.sort()
      return nums[-1] + nums[-2]