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