求大神们帮帮忙啊！！老师布置了作业我有一题不会做！题目如下

循环

int x = 5;
long y = 1;
for (int i = 1; i <= x; i++)
{
y = y * i;
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
int input;
Console.WriteLine("输入一个小于40的数：");
while(!int.TryParse(Console.ReadLine(),out input) || (input >= 40 || input <= 0))
{
Console.WriteLine("输入的数字不正确，请重新输入：");
}
Console.WriteLine(input.ToString() + "!的计算结果是：" +
Enumerable.Range(1, input).Select(i => (long)i).Aggregate((x, y) => x * y).ToString());
Console.ReadLine();
}
}
}