input: Write a python function to create a battle system where a Monster can attack a Character, reducing the Character's health by a specified amount.
  ```python
  assert battle_system(Character("Hero"), Monster("Goblin", 50), 20) == 80
  assert battle_system(Character("Warrior", 150), Monster("Dragon", 200), 50) == 100
  assert battle_system(Character("Mage", 120), Monster("Orc", 80), 30) == 90
output: class Character:
      def __init__(self, name, health=100):
          self.name = name
          self.health = health
  
  class Monster:
      def __init__(self, name, attack_power):
          self.name = name
          self.attack_power = attack_power
  
  def battle_system(character, monster, damage):
      character.health -= damage
      return character.health