input: Write a function to create a class Calculator with add and subtract methods.
  assert calc.add(10, 5) == 15
  assert calc.add(20, 30) == 50
  assert calc.subtract(10, 5) == 5
  assert calc.subtract(30, 20) == 10
output: class Calculator:
      def add(self, a, b):
          return a + b
      
      def subtract(self, a, b):
          return a - b
  
  calc = Calculator()