using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        var alf = "абвгдеёжзийклмнопрстуфхцчшщъыьэюя";

        int a = 13;

        int b = 11;

        int m = alf.Length - 1;

        var str = "скоро новый год";

        Console.WriteLine(str);

        str = str.Replace(" ", "");

        var res = "";

        for(var i = 0; i < str.Length; i++){        

            res += alf[getNumberLessMod(alf.IndexOf(str[i]) + a*i + b, m)];

        }

        Console.WriteLine(res);

        for(var i = 0; i < res.Length; i++){

            var c = getNumberLessMod(alf.IndexOf(res[i]) - a*i-b,m );

            while(c < 0){

             c += m;    

            }

            Console.Write(alf[c]);

        }

        //Console.WriteLine(getNumberLessMod(34, 32));

        //methodEiler(a, b, m);

    }

​

    public static String methodEiler(int a, int b, int m) {

        String formula = String.Format("{0}x = {1}(mod {2})", a, b, m);

        Console.WriteLine(formula);

        String mod = String.Format("(mod {0})", m);

        int countSolution = algorithmEvklid(a, m); 

        if (countSolution != 0 && b % countSolution == 0) { 

            a = getNumberLessMod(a, m); 

            b = getNumberLessMod(b, m);

            if (countSolution != 1) { 

                a /= countSolution;

                b /= countSolution; 

                m /= countSolution;

            }

            formula = String.Format("{0}x = {1}(mod {2})", a, b, m);

            Console.WriteLine(formula);

            int fx = functionEiler(m);  

            fx--;

            while (fx > 1) { 

                if (fx % 2 != 0) {

                    b *= a;

                    fx--;

                }

                a = (int) Math.Pow(a, 2); 

                fx /= 2; 

                a = getNumberLessMod(a, m); 

                b = getNumberLessMod(b, m); 

            }

            int c = getNumberLessMod((a * b), m); 

            for (int i = 0; i < countSolution; i++) {

            Console.WriteLine(string.Format("x={0}{1}", (m * i + c), mod));

            }

        } else {            

            Console.WriteLine("решений нет");

            return "";

        }

        return "";

    }

    public static int getNumberLessMod(int number, int mod) {

        while (Math.Abs(number) > mod) {

            if (number < 0) {

                number += mod;

            } else {

                number -= mod;

            }

        }

        return number;

    }

    public static int algorithmEvklid(int a, int b) {

        int d = 0;

        while (b != 0 && a != 0) {

            if (a > b) {

                a %= b;

            } else {

                b %= a;

            }

            d = a + b;

        }

        return d;

    }

    public static int functionEiler(int x) {

        int countPrimeNumber = 0;

        if (x > 0) {

            for (int i = 1; i < x; i++) {

                if (algorithmEvklid(i, x) == 1) {

                    countPrimeNumber++;

                }

            }

        }

        return countPrimeNumber;

    }

​

}