input: Write a function to generate a Python program that adds two numbers and then doubles the result. Return the final result.
  assert double_add(2, 3) == 10
  assert double_add(4, 5) == 18
  assert double_add(0, 0) == 0
  assert double_add(-1, 1) == 0
  assert double_add(-2, -3) == -10
output: def double_add(a, b):
      return (a + b) * 2