using System;

using System.Linq;  

using System.Collections;

public class Program

{

    public static void LongestPalindrome(string str)

    {

        int n = str.Length;

        int maxLength=1,start=0;

        for(int i=0;i<n;i++)

        {

            for(int j=0;j<n;j++)

            {

                int flag=1;

                for(int k=0;k<(j-i+1)/2;k++)

                {

                    if(str[i+k]!=str[j-k])

                        flag=0;

                    if(flag==1 && (j-i+1)>maxLength)

                    {

                        start=i;

                        maxLength=j-i+1;

                    }

                }

            }

        }

        for(int i=start;i<=start+maxLength-1;i++)

        {

            Console.Write(str[i]);

        }

    }

    public static void Main()

    {

        LongestPalindrome("advdf");

    }

}