using System;

namespace SqliEchallengeLongestPalindromicSubstring
{
    static class Solution
    {
        public static string LongestPalindromicSubstring(string s)
        {
            int strLength = s.Length;
            int resultLength = 0;
            int resultStart = 0;
            if (strLength < 2)
            {
                return s;
            }

            for (int midPoint = 0; midPoint < strLength - 1; midPoint++)
            {
                /*
                 * We start by checking if a substring in a palindrome from its middle.
                 * The first call to ExpandRange() takes the first case which is when our substring has a
                 * middle point i.e: "sqlilqs". The second one treats the case of when the substring's length
                 * is even i.e: "sqliilqs".
                 */
                ExpandRange(s, midPoint, midPoint, ref resultLength, ref resultStart);
                ExpandRange(s, midPoint, midPoint + 1, ref resultLength, ref resultStart);
            }

            return s.Substring(resultStart, resultLength);
        }

        static void ExpandRange(String str, int left, int right, ref int resultLength, ref int resultStart)
        {
            /*
             * While the characters starting from the middle on each side are equal, we keep expanding the substring.
             */
            while (left >= 0 && right < str.Length && str[left] == str[right])
            {
                left--;
                right++;
            }
            /*
             * If the resultLength is bigger than the range of the palindrome, we return from the function.
             */
            if (resultLength >= right - left - 1) return;
            resultStart = left + 1;
            resultLength = right - left - 1;
        }
    }

    class Program
    {
        /// <summary>
        /// The application expects a string as input
        /// </summary>
        /// <param name="args"></param>
        public static void Main(string[] args)
        {
            string stringToCheck = Console.ReadLine();
            Console.WriteLine(Solution.LongestPalindromicSubstring(stringToCheck));
        }
    }
}