input: Write a function that adds two integers and doubles the result. Return the final result.
  
  assert double_add(1, 2) == 6
  assert double_add(3, 4) == 14
  assert double_add(0, 0) == 0
  assert double_add(-1, 1) == 0
  assert double_add(-2, -2) == -8
  
output: def double_add(a, b):
      return (a + b) * 2