给定一个年月值，返回上个年月值，格式为：YYYY.MM string类型

 class Program
    {

        public string Lastmonth(string   s )
        {
           int  a =int.Parse(s.Substring(0,4));
           int  b =int .Parse( s.Substring(4, 2));
            if (b<=0||b>12 )
            {
                return "请输入正确地月份";
            }
            else if (b == 1)
            {
                a = a - 1;
                b = 12;
                return a.ToString() + b.ToString();
            }
            else if (b == 12 || b == 11)
            {
                b = b - 1;
                return a.ToString() + b.ToString();
            }
            else
            {

                b = b - 1;
                return a.ToString() +"0"+ b.ToString();
            }

        }
        static void Main(string[] args)
        {
            Console.WriteLine("年月：");
            string ss = Console.ReadLine();

            Program pg = new Program();

         Console.WriteLine(  pg.Lastmonth(ss));
         Console.ReadLine();

        }
    }