input: def sum_of_even_numbers(n: int) -> int:
      """Calculate the sum of all even numbers less than or equal to n.
      >>> sum_of_even_numbers(10)
      30
      >>> sum_of_even_numbers(0)
      0
      >>> sum_of_even_numbers(1)
      0
      >>> sum_of_even_numbers(15)
      56
      """
output:     total = 0
      for i in range(0, n + 1, 2):
          total += i
      return total