static void Main(string[] args)
{
   Console.WriteLine(Program.Calculate(5, 1));
}
public static decimal Calculate(int x, byte taskNumber)
{
   var tasks = new List<Task<decimal>>();

   for (int i = 0; i < x; i += (x / taskNumber))
   {
        int step = i;
        tasks.Add(Task.Run(() =>
        {
            int right = (step + x / taskNumber) > x ? x : (step + x / taskNumber);
            return ChunkE(step + 1, right);
        }));
    }

    Task.WaitAll(tasks.ToArray());

    return tasks.Select(t => t.Result).Aggregate(((i, next) => i + next));
    }

public static decimal ChunkFactorial(int left, int right)
{
    //Console.WriteLine("ChunkFactorial Thread ID :" + Thread.CurrentThread.ManagedThreadId);
    if (left == right)
    {
        return left == 0 ? 1 : left;
    }
    else
    {
        return right * ChunkFactorial(left, right - 1);
    }
}

public static decimal ChunkE(int left, int right)
{
    if(left == right)
    {
        return left == 0 ? 1 : left;
    }
    else
    {
        return ((3 * right) * (3 * right) + 1) / ChunkFactorial(left, right) + ChunkE(left, right - 1);
    }
}