input: Write a function to calculate the sum of all even numbers less than or equal to n.
  assert sum_of_even_numbers(6) == 12
  assert sum_of_even_numbers(1) == 0
  assert sum_of_even_numbers(10) == 30
  assert sum_of_even_numbers(0) == 0
  assert sum_of_even_numbers(-4) == 0
output: def sum_of_even_numbers(n):
      return sum(i for i in range(0, n + 1, 2))