using System;

using System.Collections.Generic;

using System.Linq;

public static class Program

{

    public static void Main()

    {

        var arr = Enumerable.Range(1, 10);

        var arr2 = arr.InsertEvery(4, "ads");

        foreach(var item in arr2)

        {

            Console.Write($"{item} ");

        }

    }

​

    public static IEnumerable<T> InsertEvery<T>(this IEnumerable<T> source, int interval, T item)

    {

        var list = source.ToList();

        if (list.Count <= interval){

            list.Add(item);

            return list;

        }

​

        for (var i = interval; i <= list.Count; i += interval + 1)

        {

            list.Insert(i, item);

        }

​

        return list;

    }

}