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