// Write a program that inputs integer n>0 and calculates

// 1! + 2! + 3! +… + n!

​

using System;

using System.Diagnostics;

public class Program

{

    public static int InputInt(string prompt)

    {

        Console.Write("Enter " + prompt + ": ");

        return Convert.ToInt32(Console.ReadLine());

    }

    public static void Main(string[] args)

    {

        int n = InputInt("n");

        Trace.Assert(n > 0);

        int r = 1;

        int i = 1;

        int q = 1;

        // invariant: r=i! and q=1!+...+i! and 1<=i<=n  

        while (i<n)

        {

            // r = i! and i<n

            i += 1;

            // r = (i - 1)! and q = 1!+...+(i-1)! and i<=n

            r *= i;

            // r = (i - 1)*i = i! and q = 1!+...+(i-1)! and i<=n

            q += r;

            // r = i! and q = 1! +...+ i! and i<=n

        }

        // r = i! and q = 1! + ... + i! and i=n => q = 1! + ... + n!

        Console.Write("...+{0}! = {1}", n, q);

    }

}