input: Write a function to add two integers and return the result. If either of the integers is None, return 0. If both integers are None, return -1.
  assert add_integers(3, 4)==7
  assert add_integers(None, 4)==0
  assert add_integers(3, None)==0
  assert add_integers(None, None)==-1
  assert add_integers(10, -5)==5
output: def add_integers(a, b):
      if a is None and b is None:
          return -1
      if a is None or b is None:
          return 0
      return a + b