input: Write a function to accept a list of 15 integers and return the sum of these integers. If the list is empty or not exactly 15 integers, return -1.
  assert sum_nums([1, 5, 9, 0, 2, 4, 7, 8, 6, 10, 3, 11, 12, 13, 14])==105
  assert sum_nums([])==-1
  assert sum_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14])==-1
output: def sum_nums(lst):
      if len(lst) != 15:
          return -1
      return sum(lst)