input: Write a python function to swap two numbers without using a third variable.
  assert swap_two_numbers(6, 10) == (10, 6)
  assert swap_two_numbers(-5, 3) == (3, -5)
  assert swap_two_numbers(0, 100) == (100, 0)
output: def swap_two_numbers(a, b):
      a = a + b
      b = a - b
      a = a - b
      return a, b