input: Write a function to generate a Python program that adds two numbers and then doubles the result.
  assert double_add(3, 4) == 14
  assert double_add(0, 0) == 0
  assert double_add(-1, 1) == 0
  assert double_add(-3, -3) == -12
output: def double_add(a, b):
      return (a + b) * 2