input: def dot_product(list1, list2):
      """ Calculate the dot product of two lists.
      >>> dot_product([1, 2], [3, 4])
      11
      """
output:     if len(list1) != len(list2):
          raise ValueError("The two lists must have the same length.")
      
      return sum(x * y for x, y in zip(list1, list2))