input: Write a function that takes two numbers as input, adds them together, and then doubles the result. If either of the input numbers is not provided, the function should return -1.
  assert double_add(3, 5) == 16
  assert double_add(0, 0) == 0
  assert double_add(-1, 1) == 0
  assert double_add(2, None) == -1
output: def double_add(a, b):
      if a is None or b is None:
          return -1
      return (a + b) * 2