input: Write a function to read in a list of 15 integers and return the sum of them. If the list does not contain exactly 15 integers, return an error message.
  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, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12, -13, -14, -15]) == -120
  assert sum_nums([1, 2, 3]) == "Error: List must contain exactly 15 integers."
output: def sum_nums(lst):
      if len(lst) != 15:
          return "Error: List must contain exactly 15 integers."
      return sum(lst)