input: Write a function that takes two parameters and returns their sum. The function should be named add, and it should accept two arguments, x and y, and return the sum of x and y.
  
  def add(x, y):
   """Returns the sum of x and y"""
   return x + y
  
  assert add(1, 2) == 3
  assert add(-1, 1) == 0
  assert add(0, 0) == 0
  assert add(3.5, 2.5) == 6.0
  assert add(-5, -3) == -8
output: def add(x, y):
      return x + y