using System;
using System.Text.RegularExpressions;

namespace Krisi
{
    class Program
    {
        static void Main(string[] args)
        {
            string value = change("draganpenchogosho", "pencho", "dragan", "gosho");
            if(value != null)
            {
                string awnser = value.Substring(1, value.Length - 2);
                Console.WriteLine("The given substring is: " + awnser);
            }
            else
            {
                Console.WriteLine("There is no such match!");
            }
        }
        static string change(string S, string X, string A, string B)
        {
            string regex = "[" + A + "|" + B + "]" + X + "[" + A + "|" + B + "]";
            if (S.Contains(X))
            {
                Match match = new Regex(regex).Match(S);
                string value = match.Value;
                return value;
            }
            else
            {
                return null;
            }
        }
    }
}