Also what here it is wrong??? You see only latin letters!!
using System;
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();

            if (s != null)
            {
               int[] s1 = new int[s.Length];
                int[] s3 = new int[s.Length];
                string s2 = "abcdefghijklmnopqrstuvwxyz";
                int tmp = 26;
                for (int i = 0; i < s.Length; i++)
                {
                    if (i == 0)
                    {
                        s1[i] = s2.IndexOf(s[i]);
                        if (s1[i] <= 4)
                        {
                            s1[i] += 26;
                        }
                    }
                    else
                    {
                        s1[i] = s2.IndexOf(s[i]);
                        while (s1[i] < s1[i - 1])
                        {
                            s1[i] += tmp;
                        }
                    }
                }
                tmp = s1[1];
                int tmp1;
                s1[1] -= s1[0];
                for (int i = 2; i < s1.Length; i++)
                {
                    tmp1 = s1[i];
                    s1[i] -= tmp;
                    tmp = tmp1;
                }

                s = "";
                s1[0] -= 5;
                for (int i = 0; i < s1.Length; i++)
                {
                    s += s2[s1[i]];
                }
                           }
           Console.WriteLine(s);

        }
    }