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