请问应该如何修改下面的代码才能实现图片所示功能？

n = 1
i = 1
user = []
while i < 100:
    letter = input('欢迎来到注册系统，退出请输入q，注册请输入r：')
    if letter == 'r':
        for n in range(100):
            register = {}
            print('请用户开始注册!')
            k1 = input('请输入用户名：')
            k2 = input('请设置密码：')
            register['name'] = k1
            register['pwd'] = k2
            user.append(register)
            if user[:n]['name'] == user[:n-1]['name']:
                print('用户名已存在！')
                break
            else:
                print('用户添加成功！')
                print(user)
                break
        break
    elif letter == 'q':
        print('退出程序！')
        break
    else:
        print('请重新选择!')
i += 1

def main():
    users = [{'name' : '张三', 'pwd' : 123}, {'name' : '李四', 'pwd' : 123}]
    while True:
        a = input('欢迎来到注册系统，退出请输入q，注册请输入r：')
        if a == 'q':
            break
        elif a == 'r':
            print('请用户开始注册!')
            username = input('请输入用户名：')
            pwd = input('请设置密码：')
            dic = {'name' : username, 'pwd' : pwd}
            for i in users:
                if i['name'] == dic['name']:
                    print('用户名已存在')
                    break
            else:
                print('用户添加成功')
                users.append(dic)
                print(users)
        else:
            print('请重新选择!')

if __name__ == '__main__':
    main()