input: Write a function to find the largest sum of two distinct elements in a list.
  assert find_largest_sum([1, 2, 3, 4, 5]) == 9
  assert find_largest_sum([10, 20, 30, 40, 50]) == 90
  assert find_largest_sum([5, 7, 2, 9, 1]) == 16
  assert find_largest_sum([100, 200, 300, 400]) == 700
output: def find_largest_sum(lst):
      if len(lst) < 2:
          return None
      lst.sort(reverse=True)
      return lst[0] + lst[1]