using System.Collections.Generic;

using System;

​

public class Program

{

    public static void Main()

    {

        bool d = true;

        int endpoint = 1, offset = 3, iteration = 0;

        List<int> list = new List<int>{1, 4, 7, 10, 11, 12, 13, 16, 19, 22, 23, 25, 26};

        while (d)

        {

            for (int i = endpoint; i < list.Count; i++)

            {

                if (i < offset + endpoint)

                {

                    if (list[i] == ((list[i - 1]) + offset))

                    {

                        Console.WriteLine(iteration + " : " + list[i]);

                    }

                    else{Console.WriteLine(iteration + " : No match!");}

                }

            }

            if(endpoint + offset >= list.Count)d=false;

​

            endpoint += offset;

            iteration++;

        }

    }

}