using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var input = new List<DateTime>();

        //Play with months here.

        input.Add(DateTime.Today);

        input.Add(DateTime.Today.AddMonths(1));

        input = input.OrderBy(i => i.Year).ThenBy(i => i.Month).ToList();

        DateTime reference = input.First();

        foreach (var date in input)

        {

            if (reference != date) //we have amethod that checks only by month and year;

                throw new Exception("Cannot have gap months on the priced months.");

            reference.AddMonths(1);

        }

    }

}