input: Write a function to calculate the sum of all even numbers less than or equal to n. If the given number is negative then return 0.
  sum_of_even_numbers(5) == 6
  sum_of_even_numbers(10) == 30
  sum_of_even_numbers(0) == 0
  sum_of_even_numbers(-3) == 0
output: def sum_of_even_numbers(n):
      if n < 0:
          return 0
      return sum(i for i in range(0, n + 1, 2))