新手小白求解n的阶乘之和的问题！当输入n为6时，结果为什么是1！+2！+…+6！=8？
using System;

namespace 实验EX1_1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("请输入正整数n");
            String str = Console.ReadLine();
            int n = Convert.ToInt32(str);
            int sum = 0,i,j;
            for (i = 1; i <= n; i++) ;
            {
                int flag = 1;
                for (j = 0; j <= i; j++) ;
                {
                    flag *= j;
                }
                sum += flag;
            }
            Console.WriteLine("1!+2!+...+{0}!={1}", n, sum);
            Console.ReadLine();
        }
    }
}

我知道了，谢谢！