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