input: Write a python function to find the second smallest number in a list.
  assert find_second_smallest([9, 7, 4, 8, 2]) == 4
  assert find_second_smallest([12, 13, 11, 15, 14]) == 12
  assert find_second_smallest([1, 2, 3, 4, 5]) == 2
output: def find_second_smallest(lst):
      if len(lst) < 2:
          return None
      first, second = float('inf'), float('inf')
      for num in lst:
          if num < first:
              first, second = num, first
          elif first < num < second:
              second = num
      return second