input: Write a function to calculate the sum of a list of 15 integers. The function should take a list as input and return the sum of all the integers in the list. If the input list has more or less than 15 integers, return -1 as an error indicator.
  
  def sum_15_nums(nums):
      if len(nums) != 15:
          return -1
      return sum(nums)
  
  assert sum_15_nums([1, 5, 9, 0, 2, 4, 7, 8, 6, 10, 3, 11, 12, 13, 14]) == 95
  assert sum_15_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]) == 120
  assert sum_15_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]) == -1
  assert sum_15_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]) == -1
  assert sum_15_nums([]) == -1
output: def sum_15_nums(nums):
      if len(nums) != 15:
          return -1
      return sum(nums)