input: Write a function to read in 15 integers and return their sum. If the input array does not contain 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, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]) == -1
  assert sum_nums([10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150]) == 1155
output: def sum_nums(nums):
      if len(nums) != 15:
          return -1
      return sum(nums)