06_CalculateSum

using System;

/*
 * 6. Write a program that, for a given two integer numbers N and X,
 * calculates the sumS = 1 + 1!/X + 2!/X2 + … + N!/XN
 */
class CalculateSum
{
    static void Main()
    {
        decimal n;
        decimal x;
        decimal sum = 0.00m;
        decimal factoriel = 1.00m;
        decimal power = 1.00m;

        Console.Write("N = ");
        string strN = Console.ReadLine();

        Console.Write("X = ");
        string strX = Console.ReadLine();

        if (!decimal.TryParse(strN, out n))
        {
            Console.WriteLine("Invalid number: {0}", strN);
        }
        else if (!decimal.TryParse(strX, out x))
        {
            Console.WriteLine("Invalid number: {0}", strX);
        }
        else
        {
            for (int i = 1; i <= n; i++)
            {
                factoriel *= i;
                power *= x;

                sum += factoriel / power;
            }

            Console.WriteLine("The sum is: {0}", 1 + sum);
        }
    }
}