input: Write a function to generate a Python program that adds two numbers then doubles the result. If the input numbers are integers, the function should return an integer; if they are floats, the function should return a float.
  assert double_add(2, 3) == 10
  assert double_add(0, 0) == 0
  assert double_add(1.5, 2.5) == 8.0
  assert double_add(-1, -1) == -4
  assert double_add(-2, 2) == 0
output: def double_add(a, b):
      return (a + b) * 2