用集合求平均分

Console.WriteLine("输入人数");
            int n = Convert.ToInt32(Console.ReadLine());

            ArrayList chengji = new ArrayList();//建成绩集合
            for (int i = 0; i < n; i++)
            {
                Console.WriteLine("请输入成绩");
                chengji.Add( Convert.ToInt32(Console.ReadLine()));//把输入的成绩放到成绩集合里
            }
            int sum=0;
            foreach (int a in chengji)
            {
                sum = sum + a;               
            }
            Console.WriteLine("总分是"+sum);
            int avg = sum / n;
            Console.WriteLine("平均分是"+avg);