input: Write a function to read in a list of 15 integers and return the sum of them. If the list is not exactly 15 integers long, return -1 to indicate an error.
  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]) == -1
  assert sum_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]) == -1
  assert sum_nums([]) == -1
output: def sum_nums(nums):
      if len(nums) != 15:
          return -1
      return sum(nums)