input: Write a function to read in 15 integers and return their sum. If the input list contains fewer or more than 15 integers, the function should return -1 to indicate an error.
  def sum_15_ints(nums):
      if len(nums) != 15:
          return -1
      total = 0
      for num in nums:
          total += num
      return total
  
  assert sum_15_ints([1, 5, 9, 0, 2, 4, 7, 8, 6, 10, 3, 11, 12, 13, 14]) == 105
  assert sum_15_ints([2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30]) == 270
  assert sum_15_ints([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]) == -1
  assert sum_15_ints([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]) == -1
output: def sum_15_ints(nums):
      if len(nums) != 15:
          return -1
      total = 0
      for num in nums:
          total += num
      return total