input: Write a function to sum even numbers within a given range.
  assert sum_even_numbers(2, 10) == 30
  assert sum_even_numbers(1, 5) == 6
  assert sum_even_numbers(0, 20) == 110
output: def sum_even_numbers(start, end):
      return sum(i for i in range(start, end + 1) if i % 2 == 0)