input: Write a function to calculate the sum of a list of 15 integers and return the result. The function should take a list of integers as input and return the sum of all the integers in the list. If the list does not contain exactly 15 integers, the function should raise an exception.
  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
output: def sum_nums(lst):
      if len(lst) != 15:
          raise Exception("List must contain exactly 15 integers")
      return sum(lst)