请人帮我完善一下以下程序:
using System;

namespace exercise4._2
{
    /// <summary>
    /// Class1 的摘要说明。
    /// </summary>
    class Account
    {
        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        ///
        int _balance;
        private Account()
        {
            _balance = 2000;
        }
        private Account(int b)
        {
            _balance= b;
        }
        [STAThread]
        static void Main(string[] args)
        {
            string choice;
            int dollar;
            int residual;
            Account ac = new Account();
            Console.WriteLine("银行初始存款有{0}",ac._balance);
            Console.WriteLine("请问要存款还是要取款？(y=存款/n=取款)");
            choice = Console.ReadLine();
            if(choice == "y")
            {
                Console.WriteLine("请问要存入多少美圆？");
                dollar = int.Parse(Console.ReadLine());
                residual = ac._balance+dollar;
            }
            else
            {
                Console.WriteLine("请问要取出多少美圆？");
                dollar = int.Parse(Console.ReadLine());
                residual = ac._balance-dollar;
            }
            Account acc = new Account(residual);
            if(residual > 0)
            {
                Console.WriteLine("剩余存款有：{0}",acc._balance);
            }
            else
            {
                Console.WriteLine("余额不足");
            }
            //
            // TODO: 在此处添加代码以启动应用程序
            //
        }
    }
}
要加入判断输入的是否是N,如果输入是其他字符那就跳出提示,我用ELSE IF做但老提示说residual未赋值,请人教教我,谢谢了~~

谢谢啊,但加了
else
            {
                return;
            }
后如果输入的不是N或者Y他就自动结束程序了,要让他再跳回来怎么弄啊?

谢谢啊