input: Write a python function to calculate the sum of a list of numbers.
  assert sum([1, 2, 3, 4, 5]) == 15
  assert sum([10, 20, 30, 40, 50]) == 150
  assert sum([0, -1, -2, -3, -4]) == -10
output: def sum(numbers):
      total = 0
      for number in numbers:
          total += number
      return total