input: Write a python function to find the maximum sum of pairs in an array.
  assert find_max_sum([1, 2, 3, 4, 5, 6]) == 11
  assert find_max_sum([10, 20, 30, 40, 50]) == 90
  assert find_max_sum([5, 5, 5, 5, 5]) == 10
output: def find_max_sum(arr):
      if len(arr) < 2:
          return 0
      arr.sort()
      return arr[-1] + arr[-2]