循环操作练习，用户输入3次0-20之间的数字，累加求和

/让用户循环操作，用户输入一个正整数（0-20），如果小于0或大于20都提示输入错误，

             //如果输入的是0-20之间的数，那么就打印从0到这个数的累加求和，

                //一共需要用户输入3遍，输入错误不计入这3遍操作当中

            int count = 0;

            for (int k = 1; k >=-1;k++ )

            {

                Console.Write("请输入一个正整数（0-20）：");

                int a = Convert.ToInt32(Console.ReadLine());

                   if(count>=3)

                {

                       break;

                }

                if (a < 0 || a > 20)

                {

                    Console.WriteLine("输入有误！");

                }

 

                else

                {

                    int sum = 0;

                    for (int i = 1; i <= a; i++)

                    {

                        sum += i;

                        count++;

                    }

                   

                    Console.WriteLine(sum);

                }

            

            }

            Console.ReadLine();