using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

{

    List<int> list1 = new List<int> { 2, 6, 1, 8, 9, 4, 12, 24, 23, 3, 11, 15 }; 

    List<int> list2 = new List<int> { 6, 9, 4, 12, 24, 23, 5, 16, 18, 2, 7, 14 };

    CommonSublist(5, list1, list2).Dump();

}

​

    private static List<int> CommonSublist(int length, List<int> list1, List<int> list2) {

        for (int i=0;i<list1.Count-length;i++) {

            for (int j=0;j<list2.Count-length;j++) {

                List<int> output = new List<int>();

                for (int k=0;k<length;k++) {

                    if (list1[i+k]==list2[j+k]) {

                        output.Add(list1[i+k]);

                    }

                    else {

                        break;

                    }

                }

                if (output.Count == length) {

                    return output;

                }

            }

        }

        return null;

    }

}