初学者if判断语句

 看到一个题：

提示用户输入用户名、密码

用户名=admin；

密码=8888；

判断用户名、密码是否输入正确，若密码错误提示用户再次输入两次，若密码还是错误则结束程序！

本人初学者发挥毕生所学也就只能捣腾成这样了，老鸟勿拍砖！

望高人指点……小鸟万分感激！！

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace mima

{

    class Program

    {

        static void Main(string[] args)

        {

            string username = "admin";

            string mima = "8888";

            string a;

            string b;

            Console.Write("请输入您的用户名:");

            a = Console.ReadLine();

            Console.Write("请输入您的密码:");

            b = Console.ReadLine();

            if (username == a & mima == b)

            {

                Console.WriteLine("用户名、密码正确!");

            }

            else

            {

                if (username != a)

                {

                    Console.WriteLine("找不到用户名!");

                }

                if(mima!=b)

                {

                    Console.WriteLine("密码错误!");

                    Console.Write("请从新输入您的密码:");

                    b = Console.ReadLine();

                    if (mima == b)

                    {

                        Console.WriteLine("密码正确!");

                    }

                    else

                    {

                        Console.WriteLine("对不起,密码再次错误!");

                        Console.Write("请再次输入:");

                        b = Console.ReadLine();

                        if (mima == b)

                        {

                            Console.WriteLine("密码正确!");

                        }

                        else

                        {

                            Console.WriteLine("对不起,您已3次输入错误！再见！！");

                        }

                    }

                }

            }

        }

    }

}