input: Write a python function to sum a list of numbers.
  assert sum_nums([1, 2, 3, 4, 5, 6, 7, 8, 9]) == 45
  assert sum_nums([10, 20, 30, 40, 50]) == 150
  assert sum_nums([-1, -2, -3, -4, -5]) == -15
output: def sum_nums(nums):
      return sum(nums)