users = {
    "john": "password",
    "jane": "1234",
    "alice": "31234",
    "charlie": "1863",
    "david": "3958",
}

def login():
    for i in range(3):
        username = input("Enter username: ").strip().lower()
        password = input("Enter password: ").strip()
        
        if username in users and users[username] == password:
            return True
    
    return False

def add_user():
    username = input("Enter new username: ").strip().lower()
    if username in users:
        print(f"User '{username}' already exists.")
        return
    
    password = input("Enter password: ").strip()
    users[username] = password
    print(f"User '{username}' added successfully!")

def remove_user():
    username = input("Enter username to remove: ").strip().lower()
    if username not in users:
        print(f"User '{username}' not found.")
        return
    
    del users[username]
    print(f"User '{username}' removed successfully!")

def list_users():
    print("Current users:", list(users.keys()))

def main():
    while True:
        print("\n1. Login\n2. Add user\n3. Remove user\n4. List users\n5. Exit")
        choice = input("Enter choice: ").strip()
        
        if choice == '1':
            if login():
                print("Login successful!")
            else:
                print("Invalid username or password.")
        elif choice == '2':
            add_user()
        elif choice == '3':
            remove_user()
        elif choice == '4':
            list_users()
        elif choice == '5':
            break
        else:
            print("Invalid choice.")

if __name__ == "__main__":
    main()