{}
# Inventory Management System
inventory = set()
while True:
op = input("Add / Remove / Show / Exit: ").lower()
if op == "add":
inventory.add(input("Product ID: "))
print("Added")
elif op == "remove":
inventory.discard(input("Product ID: "))
print("Removed (if existed)")
elif op == "show":
print("Inventory:", inventory)
elif op == "exit":
break
else:
print("Invalid option")