input: Write a python function to add numbers in a list with a step of 2.
  assert add_numbers_in_list([1, 2, 3, 4, 5, 6, 7, 8]) == 20
  assert add_numbers_in_list([10, 20, 30, 40]) == 100
  assert add_numbers_in_list([5, 15, 25]) == 45
output: def add_numbers_in_list(lst):
      return sum(lst[::2])