input: Write a function to add two numbers and then double the result. The function should take two integers as input and return the final result.
  
  assert double_add(2, 3)==10
  assert double_add(5, 7)==24
  assert double_add(-1, 4)==6
  assert double_add(0, 0)==0
  assert double_add(-5, -3)==-16
output: def double_add(a, b):
      return (a + b) * 2