初學者if判斷語句

 看到一個題：

提示使用者輸入使用者名、密碼

使用者名=admin；

密碼=8888；

判斷使用者名、密碼是否輸入正确，若密碼錯誤提示使用者再次輸入兩次，若密碼還是錯誤則結束程式！

本人初學者發揮畢生所學也就隻能搗騰成這樣了，老鳥勿拍磚！

望高人指點……小鳥萬分感激！！

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("請輸入您的使用者名:");

            a = Console.ReadLine();

            Console.Write("請輸入您的密碼:");

            b = Console.ReadLine();

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

            {

                Console.WriteLine("使用者名、密碼正确!");

            }

            else

            {

                if (username != a)

                {

                    Console.WriteLine("找不到使用者名!");

                }

                if(mima!=b)

                {

                    Console.WriteLine("密碼錯誤!");

                    Console.Write("請從新輸入您的密碼:");

                    b = Console.ReadLine();

                    if (mima == b)

                    {

                        Console.WriteLine("密碼正确!");

                    }

                    else

                    {

                        Console.WriteLine("對不起,密碼再次錯誤!");

                        Console.Write("請再次輸入:");

                        b = Console.ReadLine();

                        if (mima == b)

                        {

                            Console.WriteLine("密碼正确!");

                        }

                        else

                        {

                            Console.WriteLine("對不起,您已3次輸入錯誤！再見！！");

                        }

                    }

                }

            }

        }

    }

}