input: Write a function to implement an interactive text-based RPG game in Python where the player can move around in an environment and battle monsters. The function should include a Player class, a Room class, and a Monster class. The Player class should have attributes for health, current location, and number of monsters killed. The Room class should have attributes for name, description, and a list of monsters, with a method to add monsters to a room. The Monster class should have attributes for name and health, and a method to attack the player. The function should also include a main game loop that allows the player to choose between moving to a new room or fighting monsters in the current room. If the player's health drops to 0 or below, the game should end with a defeat message.
  
  assert GameRPG().player.location=='room one'
  assert GameRPG().player.health==100
  assert GameRPG().player.monsters_killed==0
  assert len(GameRPG().room.monsters)==0
  assert GameRPG().monster.health==100
  assert GameRPG().monster.attack(GameRPG().player)==90
  assert GameRPG().start_game()==None
output: class Player:
      def __init__(self):
          self.health = 100
          self.location = 'room one'
          self.monsters_killed = 0
  
  class Room:
      def __init__(self, name, description):
          self.name = name
          self.description = description
          self.monsters = []
      
      def add_monster(self, monster):
          self.monsters.append(monster)
  
  class Monster:
      def __init__(self, name):
          self.name = name
          self.health = 100
      
      def attack(self, player):
          player.health -= 10
          return player.health
  
  def GameRPG():
      player = Player()
      room = Room('room one', 'This is the first room.')
      monster = Monster('Goblin')
      
      def start_game():
          while player.health > 0:
              print(f"You are in {player.location}. {room.description}")
              if room.monsters:
                  print(f"Monsters in this room: {', '.join([m.name for m in room.monsters])}")
              choice = input("What do you want to do? (move/fight): ").strip().lower()
              if choice == 'move':
                  player.location = 'room two' if player.location == 'room one' else 'room one'
              elif choice == 'fight':
                  if room.monsters:
                      for m in room.monsters:
                          while m.health > 0 and player.health > 0:
                              m.attack(player)
                              if player.health <= 0:
                                  print("You have been defeated!")
                                  return
                              player_attack = input("Attack the monster? (yes/no): ").strip().lower()
                              if player_attack == 'yes':
                                  m.health -= 20
                                  if m.health <= 0:
                                      print(f"You defeated the {m.name}!")
                                      player.monsters_killed += 1
                                      room.monsters.remove(m)
                  else:
                      print("No monsters to fight in this room.")
              else:
                  print("Invalid choice. Please choose 'move' or 'fight'.")
          print("Game Over. You have been defeated.")
      
      return player, room, monster, start_game