input: Write a function to determine the second smallest value in a list.
  assert second_smallest([1, 2, -8, -2, 0, -2])==-2
  assert second_smallest([1, 1, -0.5, 0, 2, -2, -2])==-0.5
  assert second_smallest([2,2])==None
output: def second_smallest(lst):
      if len(lst) < 2:
          return None
      unique_lst = list(set(lst))
      if len(unique_lst) < 2:
          return None
      unique_lst.sort()
      return unique_lst[1]