def sum_series(n, a = 0, b = 1):

  """
  Both the fibonacci series and the lucas numbers are based on an identical formula. Add a third function called sum_series with one required parameter and two optional parameters. The required parameter will determine which element in the series to print. The two optional parameters will have default values of 0 and 1 and will determine the first two values for the series to be produced.

  """
  if n == 0:
    return a
  elif n == 1:
    return b
  else:
    return sum_series(n - 1, a, b) + sum_series(n - 2, a, b)