input: Write a function to find the sum of numbers within a range.
  assert sum_range(2, 6) == 20
  assert sum_range(1, 10) == 55
  assert sum_range(0, 0) == 0
  assert sum_range(-5, 5) == 0
output: def sum_range(start, end):
      return sum(range(start, end + 1))