input: Write a python function to find the sum of all integers between two given numbers x and y (inclusive).
  assert sum_of_two(1, 5) == 15
  assert sum_of_two(3, 7) == 25
  assert sum_of_two(0, 0) == 0
  assert sum_of_two(-3, 3) == 0
output: def sum_of_two(x, y):
      return sum(range(x, y + 1))