input: Write a python function to add two numbers without using the plus operator.
  assert add_without_plus_operator(3, 4) == 7
  assert add_without_plus_operator(10, 5) == 15
  assert add_without_plus_operator(0, 0) == 0
  assert add_without_plus_operator(-1, 1) == 0
output: def add_without_plus_operator(a, b):
      while b != 0:
          carry = a & b
          a = a ^ b
          b = carry << 1
      return a