input: Write a function to calculate the sum of 15 integers. If the input list is not of length 15, return an error message. If the input is valid, return the sum of the integers.
  def sum_nums(nums):
      if len(nums) != 15:
          return "Error: Input list must contain exactly 15 integers."
      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])==115
  assert sum_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15])==120
  assert sum_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12])=="Error: Input list must contain exactly 15 integers."
  assert sum_nums([])=="Error: Input list must contain exactly 15 integers."
output: def sum_nums(nums):
      if len(nums) != 15:
          return "Error: Input list must contain exactly 15 integers."
      total = 0
      for num in nums:
          total += num
      return total