厦门大学线下编程比赛第一题：求和

 static long next(long now, long a)
        {
            return now * a;
        }   

        static long init(long a, long n)
        {
            long result = 0;
            long now = 1;//记录当前乘数值
            for (var i = 1; i <= n; i++)
            {
                now = next(now, a);
                result = result + now;
            }
            return result;
        }