input: Write a function named `add` that takes two parameters `x` and `y` and returns the sum of the two parameters.
  assert add(3, 4) == 7
  assert add(-1, 1) == 0
  assert add(0, 0) == 0
  assert add(5, -5) == 0
  assert add(10, 20) == 30
output: def add(x, y):
      return x + y