input: Write a function to read in a list of 15 integers and return their sum. If the input list does not contain exactly 15 integers, return None.
  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([1])==None
  assert sum_nums([])==None
output: def sum_nums(nums):
      if len(nums) != 15:
          return None
      return sum(nums)