input: Write a python function to sum elements at two given positions in a list.
  assert sum_positions([1, 4, 5, 8, 9], 0, 2) == 6
  assert sum_positions([10, 20, 30, 40, 50], 1, 3) == 60
  assert sum_positions([7, 14, 21, 28, 35], 2, 4) == 56
output: def sum_positions(lst, pos1, pos2):
      return lst[pos1] + lst[pos2]