input: Write a function to calculate the sum of a list of 15 integers. Return the sum as the output of the function. If the list is empty or contains less than 15 integers, return 0.
  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])==0
  assert sum_nums([])==0
  assert sum_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15])==120
output: def sum_nums(nums):
      if len(nums) != 15:
          return 0
      return sum(nums)