input: Write a function to calculate the sum of all even numbers less than or equal to n.
  
  def sum_of_even_numbers(n):
      result = 0
      for i in range(2, n+1, 2):
          result += i
      return result
  
  assert sum_of_even_numbers(10) == 30
  assert sum_of_even_numbers(0) == 0
  assert sum_of_even_numbers(1) == 0
  assert sum_of_even_numbers(5) == 6
  assert sum_of_even_numbers(2) == 2
output: def sum_of_even_numbers(n):
      result = 0
      for i in range(2, n+1, 2):
          result += i
      return result