input: Write a function to generate a Python program that adds two numbers and doubles the result.
  
  assert double_add(2, 4)==12
  assert double_add(0, 0)==0
  assert double_add(-1, 1)==0
  assert double_add(3, -3)==0
  assert double_add(-5, -5)==-20
output: def double_add(a, b):
      return (a + b) * 2