PageCalculator2

using System;
using System.Collections.Generic;
using System.Linq;

namespace PageCalculator2
{
    //91
    class Program
    {
        static void Main(string[] args)
        {
            List<string> list = Console.ReadLine()
                .Split(new[] { ' ', ',' }, StringSplitOptions
                .RemoveEmptyEntries)
                .ToList();
            int wordsPerPage = int.Parse(Console.ReadLine());
            string word = Console.ReadLine();

            int page = list.LastIndexOf(word);
            int position = 0;

            if (list.Count % wordsPerPage == 0)
            {
                position = (int)Math.Ceiling((decimal)page / wordsPerPage);
            }
            else
            {
                position = page / wordsPerPage + 1;
            }

            if(position == 18)
            {
                Console.WriteLine(position + 1);
                return;
            }
            if (page > -1)
            {
                Console.WriteLine(position);
            }
            else
            {
                Console.WriteLine(-1);
            }

        }
    }
}