input: Write a python function to add all numbers in a list.
  assert add_nums([1, 2, 3, 4, 5]) == 15
  assert add_nums([10, 20, 30, 40]) == 100
  assert add_nums([-1, -2, -3, -4, -5]) == -15
output: def add_nums(numbers):
      return sum(numbers)