using System;

using System.Linq;

using System.Threading.Tasks;

​

public class Program

{

    public static void Main()

    {

        var total = 0;

        var pages = 4;

        var i = 0;

        var random = new Random();

        var lines = Enumerable.Range(0, 1000).Select(item => random.Next(0, 1000)).ToArray();

        var pagesPerLine = 250;

        Parallel.For(0, pages, (int currentPage) =>

        {

            Console.WriteLine($"page {++i} of {pages}");

            foreach (var line in  lines.Skip(250 * currentPage).Take(pagesPerLine))

            {

                total += lines[line];

            }

        });

        Console.WriteLine(total);

    }

}