input: Write a function to read in 15 integers and return their sum. If the list does not contain exactly 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([3, 5, 10, 1, 2])==0
  assert sum_nums([-1, -5, -9, 0, -2, -4, -7, -8, -6, -10, -3, -11, -12, -13, -14])==-105
  assert sum_nums([])==0
  assert sum_nums([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16])==0
output: def sum_nums(nums):
      if len(nums) != 15:
          return 0
      return sum(nums)