using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var res = FindMissing(new List<int>() { 4, 5, 7, 8 });

            foreach(var r in res)

            {

                Console.WriteLine(r);

            }

    }

    private static IEnumerable<int> FindMissing(IEnumerable<int> numbers)

        {

            var orderedNumbers = numbers.OrderBy(n => n);

            var min = numbers.Min();

            var max = numbers.Max();

            var allNumbers = Enumerable.Range(min, max - min + 1);

            var list = new List<int>();

            foreach(var num in allNumbers)

            {

                if (!numbers.Any(n => n == num))

                {

                    list.Add(num);

                }

            }

            return list;

        }

}