input: Write a function to read in a list of 15 integers and return the sum of them. The function should return None if the list is not exactly 15 integers.
  def sum_nums(nums):
      if len(nums) != 15:
          return None
      total = 0
      for num in nums:
          total += num
      return total
  
  assert sum_nums([1, 5, 9, 0, 2, 4, 7, 8, 6, 10, 3, 11, 12, 13, 14]) == 105
  assert sum_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]) == 120
  assert sum_nums([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]) == 0
  assert sum_nums([1, 5, 9, 0, 2, 4, 7, 8, 6, 10, 3, 11, 12, 13]) == None
  assert sum_nums([]) == None
output: def sum_nums(nums):
      if len(nums) != 15:
          return None
      total = 0
      for num in nums:
          total += num
      return total