using System;
namespace test
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Введите число для вычисления суммы факториала = ");
            int num = Convert.ToInt32(Console.ReadLine());
            double sum = 1;
            double currentNumber = 1;
 
            for(int i=2; i<=num; i++) {
               currentNumber *= i;
               sum += currentNumber;
            }
            Console.WriteLine("sum=" + sum);
            Console.ReadLine();
        }
    }
}

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Mw==

3