input: Write a function to calculate the sum of all even numbers less than or equal to a given integer n. If n is negative, return 0.
  assert sum_of_even_numbers(10)==30
  assert sum_of_even_numbers(32)==272
  assert sum_of_even_numbers(5)==6
  assert sum_of_even_numbers(0)==0
  assert sum_of_even_numbers(-5)==0
output: def sum_of_even_numbers(n):
      if n < 0:
          return 0
      return sum(i for i in range(0, n + 1, 2))