class Car:
   wheels = 4
   doors = 2
   engine = True

   def __init__(self, make, model, year=2020, gas=100):
       self.make = make
       self.model = model
       self.year = year
       self.is_moving = False
       self.gas = gas

   def use_gas(self):
       self.gas -= 50
       return self.gas

   def stop(self):
       if self.is_moving:
           print('The car has stopped.')
           self.is_moving = False
       else:
           print('The car has already stopped.')

   def go(self, speed):
       if self.use_gas():
           if not self.is_moving:
               print('The car starts moving.')
               self.is_moving = True
           print(f'The car is going {speed}.')
       else:
           print("You've run out of gas!")
           self.stop()