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(7)==12
  assert sum_of_even_numbers(0)==0
  assert sum_of_even_numbers(5)==6
  assert sum_of_even_numbers(1)==0
output: def sum_of_even_numbers(n):
   result = 0
   for i in range(2, n+1, 2):
    result += i
   return result