i=3
username = "python"
userpassword = "123456"
while True:
    if i>0：
        name = input("请输入用户名:")
        password = input("请输入密码:")
        i-=1
        if name == username and password == userpassword:
            print("登录成功!")
            break
        if name!=username:
            print("用户名不存在")
            continue
        if password!=userpassword:
            print("密码不存在")
    
print("欢迎来到贝尔编程!")