class Dog:
    species = 'mammal'
    def __init__(self,name,age):
        self.name = name
        self.age = age
    def description(self):
        return "{} is {} years old".format(self.name,self.age)
    def speak(self,sound):
        return "{} says {}".format(self.name,sound)
class RussellTerrrier(Dog):
    def run(self,speed):
        return "{} says {}".format(self.name,speed)
class Bulldog(Dog):
    def run(self,speed):
        return "{} runs {}".format(self.name,speed)
