input: Write a python function to calculate the sum of all even integers between l and r inclusive.
  assert sum_even_integers(2,5) == 6
  assert sum_even_integers(3,8) == 18
  assert sum_even_integers(4,6) == 10
output: def sum_even_integers(l, r):
      return sum(i for i in range(l, r + 1) if i % 2 == 0)