input: Write a function to find the maximum sum of any two elements in an array.
  assert max_sum([3, 10, -3, -2, 5]) == 15
  assert max_sum([-1, -2, -3, -4]) == -3
  assert max_sum([1, 2, 3, 4, 5]) == 9
  assert max_sum([100, 200, 300, 400]) == 700
output: def max_sum(arr):
      if len(arr) < 2:
          return None
      arr.sort()
      return arr[-1] + arr[-2]