Шифр Цезаря

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

namespace mainSolution
{
    class Input
    {
        private static IEnumerator<string> getin()
        {
            while (true)
                foreach (string s in Console.ReadLine().Split().Where(x => x.Length > 0))
                    yield return s;
        }

        private IEnumerator<string> inp = getin();

        public string GetString() { inp.MoveNext(); return inp.Current; }
        public int GetInt() { return int.Parse(GetString()); }
        public long GetLong() { return long.Parse(GetString()); }
        public double GetDouble() { return double.Parse(GetString()); }
    }
    static class Program
    {

        const int MOD = 'я' - 'а' + 1;
        static string Generator(string s, int shift)
        {
            shift %= MOD;
            StringBuilder res = new StringBuilder();
            for (int i = 0; i < s.Length; i++)
            {
                if (char.IsLetter(s[i]))
                {
                    char nxt = (char)(char.ToLower(s[i]) + shift);
                    if (nxt > 'я')
                        nxt -= (char)(MOD);
                    res.Append(nxt);
                }
                else
                    res.Append(s[i]);

            }
            return res.ToString();
        }


        public class Pair<T, K>
        {
            public T First { get; set; }
            public K Second { get; set; }
        }


        static List<Pair<int, int>>  countOfChar(string s)
        {
            List< Pair<int, int> > res = new List< Pair<int, int> >(100);
            for (int i = 0; i < 100; i++)
            {
                res.Add(new Pair<int,int> ());
                res[i].Second = i;
            }


            for (int i = 0; i < s.Length; i++)
            {
                char cur = s[i];
                if (char.IsPunctuation(cur) || char.IsWhiteSpace(cur))
                    continue;
                else
                    res[cur - 'а'].First++;
            }
            res.Sort((x, y) => y.First.CompareTo(x.First));
            return res;

        }


        static StringBuilder NextPermutation (string s, int letter)
        {
            int shift = letter - 'о' + 'а';
            StringBuilder res = new StringBuilder();
            for (int i = 0; i < s.Length; i++)
            {
                char cur = s[i];
                if (char.IsPunctuation(cur) || char.IsWhiteSpace(cur))
                {
                    res.Append(cur);
                    continue;
                }
                cur -= (char)shift;
                if (cur < 'а')
                    cur += (char)MOD;
                res.Append(cur);
            }
            return res;

        }
        static void Main(string[] args)
        {
            Console.WriteLine("Введите исходный текст:\n");
            Input cin = new Input();
            string s = Console.ReadLine();
            Console.Write("\nВведите свиг: ");
            int shift = cin.GetInt();
            s = Generator(s, shift);
            Console.WriteLine($"\nСдвинутая на {shift} строка:\n");
            Console.WriteLine(s);

            List<Pair<int, int>> count;
            count = countOfChar(s);
            //for (int i = 0; i < count.Count; i++)
            //{
            //    Console.WriteLine($"{i+1}   {count[i].First} --- {count[i].Second}");
            //}

            Console.WriteLine("\nВозможный ответ:\n");
            StringBuilder answer = new StringBuilder();
            int it = 0;
            while (true)
            {
                answer = NextPermutation(s, count[it++].Second);
                Console.WriteLine(answer);
                Console.WriteLine("\nif answer is correct, press \"Enter\"\n");
                ConsoleKeyInfo cki = Console.ReadKey();

                if (cki.Key == ConsoleKey.Enter)
                    break;
                Console.WriteLine("\n\nВозможный ответ:\n");

            }
        }


    }
    }