input: Write a function to calculate the sum of a list of 15 integers. If the input list contains more or fewer than 15 integers, return -1 as an error. If the list is exactly of size 15, return the sum of its elements.
  def sum_of_fifteen(nums):
      if len(nums) != 15:
          return -1
      total = 0
      for num in nums:
          total += num
      return total
  
  assert sum_of_fifteen([1, 5, 9, 0, 2, 4, 7, 8, 6, 10, 3, 11, 12, 13, 14]) == 105
  assert sum_of_fifteen([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]) == 120
  assert sum_of_fifteen([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == -1
  assert sum_of_fifteen([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]) == -1
output: def sum_of_fifteen(nums):
      if len(nums) != 15:
          return -1
      total = 0
      for num in nums:
          total += num
      return total