input: Write a function to construct a class in Python which implements two methods to add two numbers. The first method, `add_two_numbers`, should take two arguments and return their sum. The second method, `add`, should take three arguments and return their sum.
  
  assert Calculator().add_two_numbers(1, 2)==3
  assert Calculator().add(1, 2, 3)==6
output: class Calculator:
      def add_two_numbers(self, a, b):
          return a + b
      
      def add(self, a, b, c):
          return a + b + c