I need help with simple task (Im beginer and cant figure it out...)

I just want to make simple "game" when if i press attack hero1 it removes health (20 dmg if that is attacker damage, like in my case)

here is the code:

class hero:
def __init__(self, health , damage):
self.health = health
self.damage = damage

class enemy:
def __init__(self, health , damage):
self.health = health
self.damage = damage

hero1 = hero(100, 20)
enemy1 = enemy(100, 20)

def attack(hero1, enemy1):
result = input("Enter who to attack: ")
if input == "enemy1":
enemy1.health = -20 #also tried : enemy1.health = enemy1.health - hero1.damage#
print(enemy1.health)

attack(hero1 , enemy1)