input: 
  def sum_of_even_numbers(n):
      result = 0
      for i in range(2, n + 1, 2):
          result += i
      return result
  
  # Example:
  sum_of_even_numbers(10) == 30
  sum_of_even_numbers(15) == 56
  
output:     if n < 2:
          return 0