input: Write a python function to perform basic arithmetic operations (add, subtract, multiply, divide) based on user input.
  assert add(5, 3) == 8
  assert subtract(10, 4) == 6
  assert multiply(6, 7) == 42
  assert divide(8, 2) == 4
output: def add(a, b):
      return a + b
  
  def subtract(a, b):
      return a - b
  
  def multiply(a, b):
      return a * b
  
  def divide(a, b):
      return a / b if b != 0 else "Cannot divide by zero"